I am looking into some performance modifications done to my 2011 XKR.I am talking having the exhuast done,a cat back system and some other things.It is stock and it goes.Luckily my brother Charlie knows a friend of his that has done them and I have talked to him about it.Said to open the exhaust up and a cold air kit first.Plus he said not to use the E3 spark plugs,a pile of junk advertised on tv claiming performance gains.Said to leave the computer upgrades last,this is the most expensive part.Any opinions I will take.Talking having this done during the winter or next year.