I should also note that all I really care about is the Single Player.

I can run Crysis 2 SP on High smoothly, 40+ fps at all times. Looks beautiful.

However, in the Crysis 3 MP demo, I'm only getting ~30 fps on LOW. And it doesn't even look as good as Crysis 2.

Why am I getting less FPS for less graphics? Will the final build be better optimized?

I'm sure there will be improvements, but I doubt they'll be very significant at this point. More likely to see bigger gains from driver improvements. What is your hardware? The game runs great for me on all maxed settings (60+ FPS)