The issues here are, is it safe and is it legal ?
Over this side of the pond it would be legal, however as mentioned your insurance company may decline a claim if you were not up front with them.
Is it safe ? In general and with care I would agree with the above comments, yes it will be fine. The information you have about 2 strokes is pretty much correct, most 9.9/10hp outboards are exactly the same block and weight as the 15hp. So if you are buying a Yamaha/Mariner/Mercury the 10's are the same as the 15's.
I had a mad moment this summer with my 2.8m air floor tender and stuck a 25hp on it, so far i'm not dead
Take it easy, learn the limits of the boat and don't be afraid to back off and make sure the boat is always correctly inflated.
Had a look on Youtube and found a few prats with seriously over powered boats, this one made me laugh and cringe in equal measures. Remember its only a Sib and he's got a much larger and heavier 4 stroke 25hp on it.