I put one of these (The 300) on my 150CC with a 40 hp, it sure made a big difference with the low speed planing preformance, it also will get out of the hole quicker with less throttle effort.
I can run on calm water at 15-18 kts at a plane and it's best at 18-20 @ about 35-3600 rpm if memory serves.
I don't know if it knocks much off the top end preformance but with fuel here at three $$$ plus per gallon I'm rarely running at the top end anyway.
I highly recomend one.
http://www.basspro.com/servlet/cata...arTarget=browse