Well, for the Subaru community, there are a lot of hfc options. Typically, we see 10-15 hp gains at the wheels. But that's in a turbo car and usually with additional engine management. I'd imagine that on an na car you'd see a difference, but maybe not as dramatic. Basically, as the engine is an air pump, you get more air in and out of the engine more efficiently, you get more power.
My vehicle is a 2005 Outback 2.5XT (sold a 2002 WRX wagon to get it, which was heavily modded). With a bigger downpipe (no cat in it) and the COBB AccessPort I am supposed to be at around 310 hp and 360 torque. The car is a beast. There's a company called Crucial Racing coming out with a high flow cat for the car very soon and it's supposed to get an additional 10-15 whp.
Sorry, long answer to a short question
BTW, nice thing about making it more efficient up to this point is that not only is it powerful, it gets much better gas mileage. When you have to put premium in, that's a definite bonus!