You can regrind the mirror to what ever F ratio you want. Just keep in mind that it will take a bit of effort and if you are up to it you'll find plenty of people here to guide you through it. Have a look here to see what is involved. I haven't quite finished it yet but I will soon.
http://cmabar.googlepages.com/250mm%C2%A0newtonian
Regards Cristian