Pontoons are displacement hulls. They don't plane. So of the newer fancier designs, lifting strakes, third toons etc, get more of the boat up out of the water, but they don't really plane.
Tritoons with lifting strakes and large hp do "plane". I have a 25 foot with 250hp motor. It runs 44mph and handles like the 27 foot with i/o that i traded in.
To see a boat planing and making sharp turns go to thr bennington.com web site.
Note these type of pontoons list for $40,000 to $80,000.
If you want a boat that planes get a boat that is intended to plane.
Perhaps I should point out that a 250 hp motor will use about 25 gallons per hour(gph) at wot.
A 15 on a 12ft pontoon would probably do about 8 or10 mph and use about 1.5 gph.
lightly loaded 1 or 2 people
A 15 on a bare bones 18 ft utility it would probably do about 20 mph with 2 people.
I think you can see you the utility will get places faster on the same 1.5 gph.
A 12 ft pontoon has about 96 square feet a 18 ft bare utility about 108 sq ft.
These motors use 10% of the rated hp in fuel per hour @ wot regardless of what they are pushing.
Yes on the Phantom Deuce, tho I'm not sure it really planes. That said, a J-79 would make a school bus plane. But there are plenty of 'toons that plane like nobody's bidness. Here's a good example, and note that the video has a touch of naughty language in it. http://www.youtube.com/watch?v=9lSdl9ujVgo&NR=1&feature=fvwp
We have a 16 footer with a 30hp motor that goes fast enough to run ahead of weather but we want something smaller so that is my main objective. Planing doesn't really matter but it does get up on step, like a float plane. Maybe that is a better way to describe it. One ad claims that their boat with a 15hp is goes fast so that would work ok. Just need to find a used one in that category.