Just a note, you can run it up to 12 Cells! Drop the pitch (Not the diameter) of the prop, I'm sure the batteries have something to do with it - but I doubt 10mph. If you do go with a new custom pack, add a cell or 2 (For another 4200 or 8400 rpm!) if you have the space! According to Tower's specs, that motor/esc combo can draw as much as 25 continuous amps. Unless something isn't allowing that much draw (Like the batteries or the wiring).

That motor can spin WAY faster than your stock motor (Which the prop was designed for), so drop the pitch with a new prop & give the engine more of a mechanical advantage against the water, thus allowing it to spool up higher (into the power band).