Well ... not only the diameter of the exhaust gives you a certain loudness (lets call it sound ... that's better ). The straightness (or bends) of the exhaust also influences the sound of the car, as well as the material of the exhaust (stainless steel, titanium, ...)
In terms of power gains, I think they are minimal (approx 5 hp which you will not feel). In case you have a TT Supra, if you really want to add some more power to the car, remove the cats and replace it by a downpipe... tha'll give you approx 30-40 bhp more (on a N/A Supe, don't exactly know what gains to expect... maybee 15-20 bhp)
I have a RMM downpipe which I had to remove when I went for my Single upgrade...if you want it, just tell me.