185 hp out of 2.7 liters is about 68.5 hp/liter. My modded C5 puts down 418 rwhp, which works out to 492 hp at the crank out of a 5.7L engine. So that's about 86 hp/liter vs the factory 61 hp/liter.
So I got a 40% increase in power by doing a heads and cam swap along with long tube headers, cat-back, intake and computer reprogramming.
I wonder how much Suzuki leaves on the table with the 2.5/2.7 motors? I haven't looked into it, but I would imagine that an intake and perhaps even long tube headers are available for these engines through the aftermarket? Perhaps it is even possible to program the ECU?
Guess I'm getting a little carried away, but it sure would be nice the crank up the hp a little since I'll be doing all this work anyway. But I don't want to go as far as putting a turbo on it, as the car isn't all that, but a little extra gitty up (besides the move from a 2.5 to a 2.7) would be nice.