I don't think a car that floats would be a fair go - low RPM is a bit opposite to what I had in mind also. I was imagining something like the icelandic stuff, with so much power that the car could skim across the surface - no floating, just POWERRRRRRRRRRRRRRRRRRRRRRR!
What is the fastest and lightest setup available in terms of electrics? The Mamba Max and 5700kv is the fastest setup I know of, since you can run it on more than two lipo cells for insane power.
The Tamiya Scorcher paddle tyres are bolted to the three piece wheels - so there is no risk of them coming off with mega power - but mounting them to a modern chassis might be an issue I guess.
In terms of cars - well, sealing them up might be the biggest problem, and running an expensive super racing car might not be too good for its health, but who cares.
