I went from a SuperHero (which i learned in) to a Stomper 90, and the first time I took it out, i couldn't get past the second rapid at the WW center without flipping over, which i never flipped in the SH. It just takes some time to get used to the displacement hull. Try leaning forward when you go through rapids, and play with the seat position, as it makes a big difference on that boat.

Im still having trouble with flipping in the Stomper, but it gets better each time I paddle it.