the stock exhaust piping ID on a P5 is 2"s from what i remember. However, i'm not sure the muffler tip keeps that dimension. The stock muffler (look at the pic above) has a crappy tip that slides over the 'real' pipe, and is just tack welded in place close to the muffler itself. I'm guessing that too is 2" ID in there.
As said though, any competent muffler shop will get it to work. Some can do it with a flared piece that is welded inside the new tip, and then welded to the muffler...and then you won't see the reducer, as its deeper inside the tip...but the quick and cheap way will just have the reduced piping diameter just inside the tip.