One important factor would be how you are able to trim the motor. If it is mounted to the hull correctly in the first place, with the cavitation plate approximately 1 to 3 inches BELOW the keel, having it then placed at the proper trim angle or "attitude" can make a huge difference in maximum speed. For example, I have a 16' FisherMarine aluminum semi v with a 50hp Mercury 2 stroke. I hole shot with the motor trimmed all the way in and then trim out several degrees when on plane. I have GPS clocked 42 mph on that little boat but if I don't trim, it slows down drastically.
Of course, load makes a big difference too.
It's just a guess but I think if you get all the above factors lined up ie: RPM in the mid 5k range, outboard properly hung on the transom, and good trim angle, it SHOULD get up in the 20's.
The one thing about most of the smaller outboards is that the power trims (if you have one) don't have enough oomph to trim the motor out when on plane. I use an electric powered tilt with gears and it works great. It moves much slower than the hydraulic trims but is VERY powerful.