FYI, G2ergo.com's YZF-250 quick-turn throttle works on the CBR250R. It's $70 but is machined from aluminum and you can swap out the throttle cams later if you want. After loosening the throttle cables it was a direct swap and reduces throttle rotation quite a bit. I can now roll it wide open without my elbow dropping too much, which is beneficial in the twisties since it doesn't throw my body positioning off. Here's a link:
G2 Quick-Turn Throttle Tube
G2 Quick-Turn Throttle Tube