I'm doing the 700 conversion on my SH now, but I think I've heard of others using different 500 size motors in theirs.
If I'm not mistaken the stock motors are about 27 turn, an 18 turn would be faster, with less torque.
That would mean it would take you longer to get the nose out of the water, but could be faster.
There are 2 main problems with upgrading motors using the stock setup(mechanical speed control upgrade assumed) :
1. Plastic main gear can easily be torn up by stronger motors.
2. Strudder is not efficient in many ways, eats power in turns and bushings should be replaced if not the whole thing.

Saying that, if you do try different motors, replace the battery bolt pass through arrangement.
I always loved the sound of the outdrive and gearbox on the Super H but do realize it's drawbacks.
If you can't find anyone willing to talk about SH upgrades, look around for Villan upgrades.
I'm pretty sure they use dual motors and are around the same size and weight, good luck.

SB...