Being a Turbo car the MP4 is probably never going to get as loud as some of the other naturally aspirated exotics. Realistically, the Larini System can get you about a 15-20% gain in volume over the stock setup. Of course, you're also going to save weight, pickup a bit of power, keep the area a bit cooler and most importantly achieve a much more desirable tone from the exhaust.
Now if you choose to go with Larini Sport Cats, you'll increase the volume a bit further since you are replacing 4 more restrictive cats with 2 High Flow Sport Cats.
If you were to choose the Larini Race Pipes (eliminating the Cats), the volume will increase even more. You will have a Check Engine Light in this case, although the car will still perform well.
So my advice, in this case, would be to go with the Club Sport Exhaust and Race Pipes!
In regards, to performance, you're going to pickup a bit with the Exhaust but you'll really see an improvement with either the Sport Cats or Race (Bypass) Pipes installed.