I signed up on my priest when I was much lower than 650 but I'm 646 now with a 660 weapon and still haven't done LFR or normal, and I plan to do those tonight. Either way, I'm pretty sure I'll be better than the selection of Saturday morning PUGs. And you shouldn't feel bad about it either. Raid says heroic, people ought to know by now that that means ~650 for everything past Kargath.
Faizy was 'tentative' last I saw, so if he doesn't show I'll go disc. Otherwise I'll let him take disc and I'll switch to holy. My priest has no dps spec so it's no big deal either way.
That said, if there's any way to get a good healer replacement towards the end I'd like to do Ko'ragh and Imperator on the DK because Istilldon't have a fucking 670 2H and I guess it's worth blowing a couple more seals even with BRF out on Tues. Assuming we get that far, of course.