Just an opinion . . .
If you are thinking about going bit by bit to save money, why not just fit some trusted aftermarket bits like an Alta 15% pulley and a CAI? These two items would probably cost you 1k in Austrailian dollars--less if you did the work yourself. They won't change the sound of the exhaust, but you WILL get about 10 to 15 percent increase in wheel horsepower. You'll also get some really cool intake noise!
Sure, there are warranty concerns, but the 15% pulley and the CAI--especially the latter--have really stood the test of time. The only regret that I've ever heard or read about from any MINI S owner is that they didn't do these mods sooner--even before run-in.
Disclaimer: I am in no way associated with Alta (or any other car parts company). I'm just a guy who added the pulley and CAI and am very happy with it.