I put an 8500kv neu in my detta on 2s. I never took a speed reading but it was damn fast. I would guess 50-60mph, but I'm not sure.
http://www.youtube.com/watch?v=S32YX7e01cE
Jaejw1 has a Neu too. His videos should be on the related videos on the side.
I was going to get another Neu or a Medusa to run on 4s but I lost interest in the scale. You can get steel diff gears and if you have steel cvds you're set. The plastic ball diffs held up fine for me.