You do not say what size props you tried,or the make. When it comes to submerged drive ,bigger isn't usually better. Is your new prop shaft 1/8"? Or maybe 4mm?
Also lighter= more speed. If you went to two 2S lipo batteries or a single 4s lipo pack you could save a lot of weight .Longer run times too. Cheaper than nicads .
You are on the right track on cooling the motor. If you soldered copper tubes to the motor terminals and ran water through them,your cooling would be better.
n.h.schmidt