From 250W to 350W

Not exactly sure what controller you are talking about, but with in reason just about any brushless hub motor can have the voltage increased. I like to experiment a bit with different voltages through the common controllers that come with most kits. I have found them pretty tolerant of voltage increases. They are so cheap doesn't matter much if I blow one. I am currently running 48v thru my 24v 250w system without issues to this point. It just requires the ability to monitor the voltage and cut operation when the batteries low voltage is reached. This voltage increase raises the 250w motor to 816w and turns a rather sedate bike into a bit of fun.

Ideally just buy a controller at what ever voltage increase is reasonable, match the battery to it and go. However try not to over amp the system unreasonably.

Its a geared unit from an ebay seller Ebikling. I just bought a 350w 36v unit from them for my latest build. I have bought three kits from them. Nice kits and have had no issues. Price is good too. The last one was $189 shipped.

I take it pretty easy on the 250w unit on take off to spare the gears when running higher voltages. The latest build will run thru a Cycle Analyst 3. I can tailor the throttle curve to load the motor lightly on initial take off.