My old SIB was a 3.2 with an 8hp on the back and this planed very easily with two adults in, 1, 10st and the other 15st, plus fuel, kit etc etc.
3 adults and we started to struggle
, so with a 15 you should be flying
My air deck as has been said was like a "plank", to the point when i thought it couldnt take any more, but when i put the pressure gauge in it still needed a bit more puff.
I would certainly get the tuubes up to their recommended pressure, but you wont know what level your at until you get a gauge.