Alex - you're right in your views about the larger engine - no inherent harm in the extra hp in principle - BUT Philip is also right in that you will not get any of the advantages that you are expecting from the higher hp unless it is fitted with the smallest pitch, largest diameter, three bladed, propellor that you can buy for it - the standard prop assumes a light runabout capable of planing, so the pitch is large, the blade area small, and the diameter slightly less than what are typically described by outboard manufacturers as "Sailboat" propellors. The standard prop will offer little bite or grip, and will have a lot of "slip" so you will find that you still have to use high revs to get thrust, and you will need high revs to accelerate, and to go upwind in a seaway, plus have poor stopping power in astern.
If you do some web research on "Sailboat props for Outboards" you will soon find out what I'm talking about.
And even if you eventually change down to a 6-8hp, the same advice will still apply. I ran a 7.5hp L/S Johnson two stroke in the well of a Seal 22 in the 1980's, re-propped as above, and it gave me max hull speed even in a seaway at well below full throttle.
In the well, the transom height is designed for short shaft(15') rather than long shaft, but the advantage of the L/S was better grip because of deeper immersion - but the disadvantage was increased back pressure on the two stroke exhaust, so more fumes from the cooling water telltale/exhaust relief and consequential tendency for the engine to choke on its own exhaust when stationary or going astern. This in turn can be solved by modifying the exhaust relief cover and adding a flexible exhaust pipe to the transom, but that's another story ... and probably covered back in the archives of this forum.
On the other hand, out on the bracket, long shaft is good, because a short shaft is likely to cavitate as the prop comes close to the surface as the bow goes down and the sten goes up in a seaway.
So for now, my advice is focus on getting the right prop.
BTW, I hope that your 15hp is an older, lighter, two stroke and not a modern, heavy four stroke - the underwater profile of the stern of the Seal 22 was never intended to support the weight of a 15hp two stroke out on a bracket, let alone a the weight of a four stroke - the more weight stuck out behind the transom, the more the boat will squat and and just soak up all the extra hp ...
Geoff Sheddick
Parker 27/146 "Stroller'