A good rule of thimb for compression is that 1 full comp point equals approx 3%. So a 500 HP engine at 10.0:1 should be about 515 HP at 11.0:1 compression. If this 15 hp is worth a life of race fuels or octane boosters than for you it may well be a good way to increase HP a bit. You would make more power I would think in spending some time in your garage trying to smooth out the ports on your heads, or getting the machine work done properly by a reputable shop that cares about thier work. Compression is the cheap and easy way to get that last bit of power, I would try to not use it as my primary power tool :]