hi i have a 2004 ba xr8 sadan, i want upgrade my exhuast system from the stocko i currently have. i want twin tips like the new gt's and around 10% power increase. do i need to change my extractors or can i get away with a cat back system?

dont expect much power out of just a cat back.
do the extractors aswell will sound a s**t load better too.
even high flow cats if you've got the money

You could put just the catback system on no worries, but after that you'll want more grunt and wish you fitted the rest

I would go and get the whole system fitted, headers, high flow cats, and the cat back exhaust with whatever tips you want. That way you wont have to go back and get headers fitted or high flow cats in the future. And if your paying someone to fit, you'll save a bit in labour getting it all done at once i would think.