I don't remember the stock pipe diameter but I do know something about pipe sizes. People have tried pipes from 2.5" to 3", and 2.5" seems to be the best comprimise. With 2.5", you still retain power down low, and gain some in the upper rpm range. Anything bigger than that and you begin to lose low end torque. Remember, you don't want backpressure, you want velocity. With 2.5" pipe, scavenging will still be good at low rpm and it will be freer flowing at high rpm than stock. A 3" pipe would give you mininal backpressure, but also a lack of exhaust velocity, so that at low rpm, the exhaust system wouldn't be good at scavenging (moving) exhaust out of the cylinder. At high rpm, since there is more gas traveling through the system, scavenging isn't as important so a 3" pipe will give you the biggest gains up top.
Feel free to correct me if you know more about exhaust than I do
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BTW, 2.5" pipe is what I'm planning to run in my car when I get a custom exhaust done this summer.