One thing with the Jazzes is that they're not easy to cool. You can put a fan on them, but adding a heatsink is a big, warranty-voiding operation. AA, will do. :)
Cooling was the first thing I thought of when I saw the esc.
It's tough to dissapate heat when it's inside a plastic case!
The power required for a speed is not linier but goes up with a square so if you can do 40mph with 300W, 700Wwont give you 80mph it will give you just over 60mph, to do 80mph you would need 1200W