Modifying the airbox will need rejetting. It'll allow more air into the motor at high rpm which will lean things off. Consider going up at least a couple of sizes on the mains.

Dave's spot on with his plug chop theory. Heed his advice or you could run into trouble.

Let's not knock the CBR250RRRRRRR though. There's not many bikes that can cruise round town at 15,0000rpm. Oh yeah, and don't let me borrow it 'cos I tend to just hold the thing in first gear with the valves bouncing at over 20,000rpm, laughing my head off. They're fun