Firstly when your referring to exhaust, did you just change the muffler or the whole exhaust system...ie extractors, etc.
Also you lowered your car, and left the standard suspension....ouch
you'll probably find its a bit of a hard ride.
I suppose its really decision time....what do you want out of your car?
performance, handling, or just have it as a cruise car?
in terms of engine mods you have to decide whether you want to keep the car NA or turbo charge it. Although the engine is pretty good the way it is.
Also remember both types of setups involve lots of $$, and once you start, you'll find there is more things you will need..and the process will go on and on