I'll put my hand up, I'm one of those who have used a 15hp on a 8hp rated inflatable. Boat being a sunsport 270 v air deck.
Boat was new when I bought it, started with a yam 6hp, only get on the plane with one. 11mph.
Next 9.8 merc, on the plane with 2 up and some gear. 1 up 14mph.
Then 15hp merc, same weight as 9.8, the only time it felt over the top is one up,,,,,,22mph.
Days out, 2.5 up and bits of gear very user friendly.
Only time it would not get on the plane was a wild camping trip,,,, 2 up, camping gear, food, water, refreshments, 60 litres of fuel and some bags of fire wood.
The sunsport had large tubes, inflated floor and could have a 4 stroke engine 20 kg heaver than the merc. Having gone up in engine size over some years it worked well. Had the inflatable been old, smaller tubes, dodgy transom,,,, not some thing I would have done.