You'd need to give much more information about the bike before anyone can answer that. If you can move to a more powerful motor you may well need to upgrade the battery and controller as well.
If you do, it will no longer be legal to ride on the road unless you register it, get insurance, etc; and also not legal to ride off road in public places (bike paths, towpaths, bridleways, etc). Theoretical penalties are high; but risk of being caught if you still ride sensibly very low (and pretty low even if you don't ride sensibly).
Yes you can but it won't make any difference, If you want more power you need more current or voltage. Even a 250w hub can output 1000w or even a lot more with mods..
A guy in Germany hot rodded his Bafang 250w SXWK hub motor with 35a/ 60v and some engine cooling fluid for a bit over 2000w on a tadpole/bent trike. Thinking of it; it might have 40a.