Diff between 15 & 20 hp motor?

Looking for your thoughts on the Difference between a 15 Hp & 20 Hp Honda.
I know that per the Specifications they are the same motor, so the difference is in the carburetion & RPMs.
I have a Honda 15 on a Gregor 15ft Alum boat. With 750lbs of stuff in it, per my GPS it will do 19.5 mph.
What would be you guess that the 20 hp motor will do for top speed?
Will the trolling speed be the same on both motors?
Will fuel use be the same except when running at full throttle?

OK I’ll ask about stepping up to the 25 hp too.
How fast?
How Slow?
How much more fuel?

Thanks for the help.

I know that the difference on the 25 and 30 (2001 vintage) is in carburetion and camshaft. I'm not sure if the latter is lift, duration, or both. I don't think their max RPM is different.

I think you will disappointed in the performance "bump" you might get if you stepped up to the 20. I would suspect no more than 2 to 3 mph ground speed increase with a bit easier planing and small additional load capacity. Freeportd is correct that the only real differences between the two is cam lift and carburetion jetting. They are both covered under the same shop manual and even weigh the same.

The 25 hp is a totally different animal, boasting 3 cylinders and carburetors. It would be a big step up in performance for that heavy Gregor hull and might possibly even push you into the low 30 mph range all else being equal and the boat trimmed up sharp.

The 25 will be a bit harder on gas as a result and maintenance will, at some point, cost more in either shop fees or your time when those carbs need to be synced.

I love the 20 and would love to personally own one with electric start and power trim. The pure simplicity and reliability of that outboard is nothing short of poetry... IF you do the chores to keep the carbs cleaned and the fluids and filters changed. It is my favorite outboard, as a boater and a mechanic, of all time. But, you owning the 15 and all, you already know about how great they are.

I would think that all three would perform about the same at trolling speeds except the triple is going to idle a tad smoother than the twin which tends to shake a bit at low throttle.