Exhaust

A 2-1/2 open pipe can handle 600hp.
The muffler you choose to add might shorten that down a huge amount. Or not at all.

The best turbo mufflers are going to allow approximately 450hp before they become a restriction.
Then you have to look at the better chambered mufflers.
If your going to turn the screws on the engine for some serious power, look to the straight through mufflers like a Hooker Maxflow, Borla etc

A 2-1/2 is going to be quieter over a 3.