What hull and load do you plan to use the motor on?
I have a 16' Alumacraft flatbottom that is typically loaded with me at 170 lb, bow mount trolling motor, group 29 battery, 3 gal fuel, 60 lb flooring, fishing gear, plus a little bit of other random stuff....with my 1982 9.9 hp Evinrude it will do about 17 mph on glass water at 5400 rpms with an OEM 10" prop, which results in about 20% prop slip. Same load and prop but with my 1992 9.9 hp Johnson with a 15hp carb and it will do about 21 mph at 6500 rpms which is closer to 17% prop slip.
Top speed there isn't a big difference but the holeshot is much more noticeable. It is also helpful to have that extra 5 hp or so for going with multiple people. My boat will hold 3 people comfortably for crappie or catfishing and for 3 people having 15hp paired with an 8" prop is super nice.