my r.p.m. would top out a 4500 with the merc but now i am lucky to get 3800 (i am not sure about the max rpm on the jasper) and when ever i do the temp rises from 155 (which is how hot my old engine would get at top speed) to about 190 F. i have never changed the prop with the new engine i changed it about 2 years ago with the old engine. I dont know if this makes any difference but the lower unit has about a 4 inch crach in it that i noticed after the new engine was installed would this make any preformance differences. i would use another mechanic but i keep my boat a a marina and they are right at the marina and because of that they are much cheeper for labor other wise i would have had sombody else replace my engine. My g.p.s. said 36 knots i think if my math is correct that should be 41 42 mph.