Hi Chris - last item of the day. I ran the 6v 400 4.09:1 with the 10x6 HD on 8-cell KAN 950's , freshly charged.

7.0A*, 10.1v, 71W, 5340 rpm, 350g (12.32oz) static thrust, 4.93g/W

That's pretty close to what you'd get with KAN 650's too.

From my old data sheets I found a run with an 11x7 HD:

9.6A*, 10.8v, 106W, 5010 rpm, 508g (17.9oz) static thrust, 4.8g/W

That is, I think, far too much to push through a 6v 400! Its life will be "nasty, short and brutish".

Whether you go for NiMh or Lipoly, the 6v Speed 400 with a 10x6 HD seems to be a good choice.

Cheers, Phil