I'll try to answer best as I can compared to my 11 M3 and 11 135i and a few 335i and 550i I have driven and became familiar to (Families vehicle)

1. BMW are known for their performance. Their gearing and Tranny gives a better advantage, also their higher rpm and their motor it's self is top notch Racing like feel to it. It's there as it is, the drive can become a better driver by just driving a BMW. It varies by the driver, My gf who wanted an audi and Mercedes was convinced they're all about the same. but When it was my turn to test drive it with the approval of the sales, I showed her the difference between companies and there was a huge difference in the way the car performs. BMW she said felt more like a track race car vs others.

2. Real world driving compared other makers varies, as mentioned Above a BMW is there as it is, Performance is set to it's limit, rest is up to the driver to see if they can drive it to it's full potential or not.
How ever a Jag is different, It's not easy to explain till you have tried to push the jag's limit. a BMW 335i w/M and full package (2013 specifically) performed less than my 135i, In terms of Real world Performance I knew the cars applicability and it's limit, I am able to go into corners at high speed and exit out of corners at high speed (yes once in awhile i drive a little track style in normal streets but not dangerously)

A Jag though, the car is limitless to the driver, Vs. a BMW's performance.
A BMW is based on how well the driver can push it, a Jag is how do I explain this.. The car evolves and adapts to your driving ? the more aggressive you drive it the more aggressive it adapts to you, It becomes a little more technical. My gf can just floor it and take corners, a Jag you must understand the vehicle.. Gas release Gas release on corners and it'll handle the corner even better than a BMW (Yes.. surprisingly MY XF SC can take sharper and harder corners faster than my M3)
I have yet to fully understand what my Jag is capable of, And coming up MFest I will track it to see how beastly it really is. I've taken it on mountain backroads and it can handle better than the M3, Big Bear going the back way my M3 averages going 50-60 mph on the corners, my Jag was able to achieve the same without any tire sounds and exit the corners at a faster speed.

as I was driving it, I realized the car sort of speaks to me lol.. I'm not crazy, not literally but I can understand the car on part for performance. We sort of became one ?

For the Non performance , as a daily drive. it is Much smoother, and Much softer than any BMW I've owned. It was even softer than my MBZ C63.
It felt more like a S class in terms of daily drive comfort.

3. Unfortunately there is not many mods around in the market. More for the SC and R versions and much less for the Lower versions, Exhaust is basically all you have. ECU is out of the question as it will void warranty without a doubt.
Other options would be to weight deduct. Wheels , Exhaust (Stock OEM is quite heavey) and seat deletes (which i dont think you're interested in)

Remember Unsprung weight makes a big difference in Acceleration, Braking, Handling. a bigger roll than suspension upgrades. For those who do not understand how the weight deduction works, if OEM wheels are 36 Lbs and Aftermarkets are around 26lbs (just an example) that is 10lbs difference, without tires involved. how does 10lbs help ? that is 10lbs x4 (all wheels)
40 lbs is not what you're looking at, 1lbs x 10lbs = Metric weight.
the Metric weight will be how many lbs less your car needs to handle per corner and how much less weight it needs to carry on acceleration and how much less weight needed to be stopped during braking.

4. The Navi has been upgraded from 2012+ a small improvement for some but for me was a much bigger improvement, Less lag (although there still is lag)
everything works more accurately (the touches) and best of all, a better layout.

5. Yes it is possible, depending on the model you're getting, by the looks of your interest, yes it is. The headboard there is nothing special inside. It will have to be a small Screen though, or you can just have a wrap around screen onto the headboard. Car seat is easy how ever it may move around alittle. BMW has more of a bucket seat feel to the back, where the Jag is more like a Straight Bench feel.

I hoped my answers can help , I've tried to be as detailed as I can.
Excuse my poor grammar lol, and remember this is from my experience.
Not something I've read off some where or anything Official so please for others do not ask me to state proof or show links.
From my point of view I'm trying to help our fellow member as much as I can.

Now ofcourse others may disagree as we're all different, and our driving is different.

BTW I'm a "M" lover at heart, and always will praise BMW's
I personally find them to be best bang for buck on performance.

But the Jags are starting to impress me,
I was considering an M5 but was so disappointed I looked else where, Instead of the XFR as a testing I got the SC non R
I have no regrets yet.

 

That's it in a nutshell, well maybe not a nutshell but the point made is correct.

Jaguars are designed to be easy to drive quickly on ordinary roads. The faster you go the less extra effort is required compared to other Marques. Only Alfa Romeo comes close to that ease of driving combined with a comfortable ride. SAABs are easy to drive quickly also but they shine mainly on very poor roads and ride a bit firmly.

 

I think that you are taller rather than shorter syed?
I am 6'1 my wife is 6
I had a 2010 a4 before this and this is what I felt.
The a4 might have had a bit more rear leg room if not the same as the xf.
Also the a4 had more head room than the xf.
Both were a bit of a surprise to me as the xf is a much bigger car than the a4. So I expected more leg room. In comparison, I felt the a6 had a ton of rear leg room and good head room.
I loved the a4 styling. But looking at the xf you can see that's its just so damn pretty. And the roof line is much more raked at the rear.
I jumped into an Xj and felt that at the flagship level there should have been. More leg room and head room but there wasn't. Same as the xf if not a bit more in both categories. (Haven't jumped into any L models)
In closing I felt this.
I only have a 3 month old and so he's not going to care about leg room and headroom for a while.
I don't sit in the back, My passengers do. And anyone that's over 6'3 ish isn't going to be the most comfortable in almost any back seat of any car that looks like this. If you have a big tall family. This might not be the best option for you?

My mother (all 4'8 of her), knows nothing about cars, said it best: "when I was in your black car (a4) it was nice. But when you drive me around in this car I feel like royalty! Everyone looks at you differently when you get out of this car"

That's what I love about this car! Now I can't wait to get her into an Xj!

 

By the sound of how tall you and your wife is,
I have a feeling you're son will be 6'3ish in few months !

I'm no where close to that height so I wasn't able to answer the seating lol.
but to me it felt a little more leg room than a 335i, and alittle less head room than a 335i

either way for shorties like me, was plenty lol. we asians don't grow much

 

Hhaha.
I am Asian too! Parents just fed me the same stuff Yao Ming ate!
And the kid is big!! But he's a tubby! He's 3 months old and weighs 20+ lbs now!
So damn cute! He enjoys the jag too! I think!

 

LOL , My parents should've taken some notes on feeding babies.

Congrats on such a healthy baby, Babies are adorable when they are chubby.
20 lbs lol, Ill bet you can bend steel like nothing.. having to lift the baby all day.

strong vehicle, strong owner

well there you go, if the baby that big has no problem in the back seat, no other baby should !
He's probably taller than my 8 year old son