Well I rolled back to 11.1 (the only other driver on my system at the time) and I have good news and bad news...
Good news: The rollback seems to have fixed all issues I had, I'm back to peak performance! This means that the nightlys are not the source of my problem.
Bad news: This puts the blame squarely on ATI's 11.7 drivers.
May I suggest a rollback to a last known good driver for those using 11.7? It seems to have worked for me and it's no more then an hour and 4 restarts at the worst to see if it works and put it back if it doesn't.
11.7 should be ATI's latest attempt at the Mjolnir performance drivers right? I think I've read on Guru3D that the OpenGl performance on them isn't so great. Interestingly, I'm on the 10.8 preview with a 3650 HD mobility and have no slow downs to speak of, at least judging by my usual benchmarking method of using the ship lab. And I definitely don't have long mission load times.
@KyadCK: Are you just getting bad framerates, or super long mission load times as well? Did you remember that the newer builds run FXAA?
Just bad and weird framerates on 11.7. I know the nightlys have been useing FXAA for a while now, but I don't use it as I just don't see much difference in-game except perhaps a slight fps drop (perhaps I don't know what i am looking for to compare?). Regardless, the driver rollback worked and I'm seeing my usual fps ranges on my benchmark, so I'm going to write it off as the drivers and do more research before I go jumping on the new driver bandwagon in the future.