So you are asking good questions fore sure. I like this group, WW because you can find serious answers to serious question, not all sites for Powell are like this.
To get a larger engine you'll need a Tritoon to support the weight. Pontoons are only available up to 150 HP commercially and that's a recent development because the logs have gotten larger. Traditionally older pontoons have not been produced much beyond 115 Hp and yes they are dogs in the water. I've got a 25 footer with a 115 and some performance enhancing Hydrofins under it (I'm a hydrofin dealer and certified installer) and solo with a 1/2 tank of gas and on a smooth water day I can go 20/22 mph. Get a little algae on the toons, and the family, some gear and some chop and it's 14 to 16 mph.
If you go with a 22 footer and a 150 and given that you are propped right, have a light load and smooth water you can go 40. But add some alge on the toons. some rough water and a load a you'll loose a lot of speed. My buddy bought a new 22 foot Lowell with a 150 two years ago and really wishes he'd gone with a 200 hp for just those reasons.
Outboard technology and Tritons have really made the aluminum log boat experience a whole new deal. The companies building 8.5 wide boats with larger center logs and higher HP are where it's at. (the can lean into turns) For as choppy as Powell can be I say go 25 feet in length and at least 200 HP if you plan on hauling a family unit and gear. For a 22 footer I still say get a 200 too. Boats prices are down a bit but a new one will still set you back a small fortune. I've got a center log and a 200hp outboard for mine sitting at a boat yard in Page and that upgrade project is in my calendar for my boat.
Hydrofins are still a great solution for older twin pontoon boats with 115's but they never go much faster than 22 mph. The other benefits to Hydrofins are better turning, fuel savings and a smoother quieter ride. They can be a nice easy solution to some of the pontoon boat woes.
I bought my aluminum log boat 6 years ago just to check it out, I didn't now much about them. I love the type for Powell; aluminum, low maintenance, lots of deck space, lots of shade, very beachable and easy to board.
I know a guy who was selling an older JC tritoon (great company) like 8 wide and 30 long with a newer fuel injected 300 on it. I think he was asking under 30k for it. You can IM me and I'll find his info for you if that's more of your price point To me the longer the tritoon and the higher the HP the happier you'll be with it on Powell.