Well then, the most I can tell you is that any size exhaust inlet will fit a P11 G20. All you have to do is use a reducer or expander when you have it welded on to the factory piping.
Where you'll run into problems is the information you don't know. If the muffler is too long, it will probably hit some of your rear suspension and/or undercarriage. If it is too large in diameter it might also hit the undercarriage.
Just going by the inlet size will not determine if a muffler will fit your P11. You might want to email the seller to get more specifics on the muffler.
I don't know who makes this muffler. its stainless steel.
Does the canister determine the amount of sound thats produced and type of sound or the tips? Cuz I know Arospeed and HP make a muffler just like this. I want one that isn't so loud adds a little growl to my car. Mainly the looks I'm going for.