Exactly what it appears to be, a relative 33% increase (600kv). Of course due to wind resistance increasing the faster you go, its not quite that linear, but its a good indicator. Similar case with pinions, add 50% more teeth, get an effective 50% top speed gain (in a perfect world).

Then there's efficiency to consider. A more efficient motor will make slightly more torque AND less heat, that's why people buy expensive motors. If you want my recommendation, get the MMM combo 2200kv. Some will say its an MT engine, I say 'great!' Anyway, the Castle/Neu 1515Y is a very efficient motor, MASSIVE power, won't get too hot in a buggy. It won't give a crap what pinion you run on it on 4S within reason...