Take it offroad and test it, watch them temps and you should be able to know after how you stand.
It's not exactly that simple... a bigger pinion will give you more top speed but the acceleration will not be as good as it was, also it will draw much more juice from the batteries so running times will drop as well.
For what I can remember, my best setup on the hyper st was a castle 2200kv motor with a 15t pinion this was on 4s. Temps were cool and running times were very nice as well while still having plenty of speed and punch...