Hi Chaps
Please accept my apologies if this subject has been done before...

I was wondering what performance mods are available for the 4.2 XKRs, is there anything out there which doesn't compromise reliability and does not involve a loud exhaust?

Thanks in advance...

Supercharger pulley upgrades give the most bang for a relatively modest buck, The extra heat generated, though, causes diminishing returns the faster you turn the supercharger. This leads to a desire for intercooler upgrades especially at higher boosts (such as both upper and lower pulleys) and/or if you use the extra power repeatedly in a short timeframe.

Depends on how much you want to spend, for big money go for one of avos's twin screw Kenne Bell upgrades, that will see you good into 500bhp+.
Apart from that as White has already said, it's a slippery road when you go on the 'more power' route