They did some optimization for older GPUs after the hardware compatibility test, so the Framerate could as well be higher in the final game, that with the hardware compatibility build.
Having the newest drivers for your graphics card installed sometimes also helps.
It's hard to talk about playability only knowing the avg. FPS because it also depends on how smoothly these frames are distributed over time when you actually play the game.
Framerate drops can be really annoying. I'd like my avg. FPS to stay at least above 50, though i would say everything above 30 is playable.
In the end everything you're willing to tolerate is playable.
