After many attempts, trying to break 700m/s (got 699 3 times before hand!) I managed to pull off 700m/s. Granted I did use a MechJeb pod and the tool itself to keep me straight. I believe it would be very difficult to get much faster than this, though there is still a lot I don't know about the game so I'm likely wrong. I tried to get a screen shot as close as i could to it actually happening.