Yes on the tune, or you'll lose power.
Yes on the valance as XKR stuff have two pipes each side, the XKs have only one each side. Those are kind of inexpensive though.

 

​​​​​​I also would be interested in a how-to for modifying the standard 5.0 n/a exhaust. Also how good the result is with regards to both the required sound and any intrusive noise.

 

When I had my 2012 XK,k I bought and had installed a Mina Galleries back box. It came with a diffuser that would accommodate the four outlet exhaust. It is a bolt on modification other than painting the diffuser. You will like the sound and you will gain a little more horsepower. If I remember the whole thing cost about $1100.