You likely will see even more vertical performance with an 18x6 and may not lose much if any air speed as compared to the 17x8, Dan. All depends on how draggy the airframe is. At a given peak rpm a larger prop diameter creates more thrust, whereas greater prop pitch creates more air speed. Thing is, if the airframe has a lot of drag, a higher prop pitch/smaller diameter prop may not have enough thrust to drag the plane through the air any faster. I'd definitely try an 18x6, it may just tickle your fancy.