mate as a lot of people have said on here is you can do whatever you want to your car, because its your car. There has been some good advice shelled out here mate. First thing to do is sell the 3.5" exhaust to someone with a turbo and use that $$ to get yourself a nice 2.5" system which will give you a better exhaust note as well as more power.
If you love the car for what it is then by all means you wont be thinking about hurting your car would you ????