The NSR150SP has some healthy restrictions from the factory.

First you need to dump the pipe and fit a decent expansion chamber. This can grab you a bunch of ponies, 'cos the stock pipe has a cat in it and weighs a ton.

The engine is quite restricted by design, but unless you're a bit handy with the grinder then there's probably not much you can do about that.

A race CDI is another option. Designed for SP racing (standard engine & carb).

You could also junk the airbox and fit an after market filter and rejet.

If you're feeling flush then a big carb can help breathing, but anything over 34mm tend to be a little over kill unless you can modify the intake tract of the crankcases.

Basic bolt on mods could see you up around the 28hp mark. Unlikely that you'll get over 30 gee gees without engine mods.

40hp is possible with max mods to everything, big carb, etc etc.......

As to whether it'll beat a CBR250? That's a sticky one, 'cos the CBR isn't actually too bad. It'd be a close race, but only because the CBR is a fat pig. Then again, the NSR150SP isn't exactly a lightweight.

Best plan is buy the NSR150. Stick a pipe on it to give the motor a bit of freedom, pass yuor test (or whatever you have to do), and then buy an NSR 250 MC21 and demolish any CBR250RR that has the cheek to try and compete.