well, to get more than 300mm you are starting to get in to some pretty serious dosh, and I really don't think a mirror lens will give you the results. From memory, mine was about f11 as well, which was quite restrictive. Sorry to be a bit negative, if you live anywhere near Birmingham, I'd suggest a visit to the exhibition at the end of February in the NEC where you can see a lot of equipment, just google it and you'll find the info.
Sorry, the exhibition is called "Focus".