Take what I say with a grain of salt... because I don't have tides, and I don't deal with current...
I've used my 3.5 hp (29lbs) on my 25 Capri for a whole season now, and have found that even in the pounding of decent waves, and a 25 knot headwind I can still achieve 4 knots into the wind on my Johnson 3.5hp (which is also a tohatsu, and merc, and 3 other brands) outboard. Mine is a 2 stroke sailboat motor though, so is designed with a prop for displacement (the prior owner wasn't happy with it, as he put it, "it wasn't fast enough for his johnboat.")... Well DUH it's a sailboat motor! If he had tried he'd have likely found that he could have towed 3 other boats the same size as his at the same speed though!
I also had a 2.2 merc (same body style at the 3.5, but a couple years older)... I found several old mercs, and I think yamahas that were the exact same motor. It would stall completely at 20 knots headwind the boat, but with NO wind, could move the boat above 5 knots, it was designed to push a RIB or other inflatable.
I think a lot of what you are looking for is prop pitch, not HP... it's a given that the longer shaft motors will give you better bite into the water as well. If I were sailing moving water, or experienced consistent steady 15+ winds (doesn't happen by me) I'd want the long shaft purpose built sailboat motors, and on a Capri 22, a 4hp would be sufficient enough for me. Now if I sailed a river, going to a 6hp long shaft might be the way I'd go.
The 6hp short shaft Johnson I had on my Capri 22 was a super quiet and reliable 2 stroke, and would push the boat to hull speed at half-throttle on a calm day. It only weighed in at about 10 lbs heavier than the equivalent 4hp, so it wasn't a huge hit to the balance of the boat either. It also wasn't light though, at 57lbs. It DID however, have and alternator and an external fuel connector which are extremely nice to have if you are going to spend MORE than day-sails on the water.
So THERE is my advice... if you plan to cruise at all, hit any kind of chop, or sail on running water, go with a 6hp with an alternator.. get a 2 stroke (lighter weight than a 4 stroke), move some ballast forward, and get the long shaft model to bite in a chop. The added weight for the higher HP will be easy to off-set by moving your anchor in a far forward locker (or relocating your battery forward). You'll be able to put a remote tank in, and also charge your batteries. Anything over 8hp is overkill on a 22.