That's exactly my situation. Going to 720p requires half the fill rate of 1080p. Turn off some postprocessing and antialiasing and you might get double the perf

That's exactly my situation. Going to 720p requires half the fill rate of 1080p. Turn off some postprocessing and antialiasing and you might get double the performance again. So if you're at 1/4 peak performance, you're still running a smooth game albeit not quite at full detail. If you're at 1/16 peak performance... things aren't looking too good by the time you get your game smooth.