Brushless ESC's no longer have the trouble brushed speedos had when running partial speed. You can turn down the throttle by however much you like and not worry about overheating.
Brushless will overheat if you gear them wrong though. Only difference to brushed is they will overheat if geared too low as well.
If you're worried about going too quick why not use a slower motor and run it full instead of using something faster and turning it down by 50%?
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