He's got a 6.5 ROM but only to 2.02BL:
http://forum.ppcgeeks.com/showthread.php?t=77355
You can see if it fits your 2.05. That's where the kitchen comes in. You can "cook" a custom ROM to your specifications. Used to be pretty active flashing when I had my XV6700 but never went to 6.5. I stuck to a stable ROM 6.0 that I cooked and that would be my advice to you. The different WM versions really only have a few more features than previous and all can be solved with the correct 3rd party software in WM5. WM6 gave HTML email, 6.1 gave threaded SMS, and 6.5 just gives a better IE. Flashing is not an exact science and sometimes can get you into trouble. This board is not very active anymore so not much help.
That BL downgrade is in the sticky above "Stuck in BL 2.05" or something like that. It can be pretty tricky and is probably where most have bricked the phone. Usually you can get out of a bad flash. Be careful.