I have had mine for almost 3 months now. Bought it second hand (one year 4.300 miles) from someone who had obviously driven it very carefully. The rear (and front) tires and the brakes looked like they were almost not used at all.

After I have driven it 2.500 miles, it feels definitely stronger. It takes a lot less throttle input in the first two gears (specially noticable in 2. gir) before the rear tires struggle for grip. It is clearly more lively and responsive than it was when I got it. I also had it in for it's first service one month out in my ownership.

I can't say I hear any differense in sound though, but that might be because any possible changes has come very gradually?

The notice of change in power/throttle respons is very noticable though.

 

Yep, v8s convertible.
It was really unsettling. I felt like it was quieter, but there was nothing wrong with the exhaust according to the mechanics. I thought I'd gotten used to the sound of the car and no longer found it as zesty. I was a but sad.
I didn't notice anything changing after that, it just subtly went back to its usual loudness. I didn't even think about it till I opened this thread a couple of times.