The levels after the demo level are a little more demanding and also a little larger and I guess the AA hit is larger as well. So far I've been playing at 1080p (with 4AF, no AA) with Gaco's 1.1 config and just started the Awakening level. Performance has been always playable and enjoyable, no major difference to the demo level performance wise. High levels of AF and AA most likely do cause a bigger performance hit in these levels. Try the levels with lower amount of AA (or none) and it should perform similar like the demo level and if it doesn't there might indeed be an issue somewhere.
What are your ingame settings or do you use a custom config, whats your driver and OS?
Intel I7-920 aircooled by Prolimatech Megahalems @ 3.8Ghz (200x19) | Gigabyte GA-X58-UD3R | OCZ Platinum 3x2GB DDR3-1600 RAM (7-7-7-24) @ 1600 | Gainward GTX 480 | WD Raptor 74GB + 150GB + Samsung 1TB | Soundblaster X-Fi Platinum | Corsair AX850 Gold | Coolermaster Stacker 830 silver | Logitech Z-5500 Digital 5.1 | Logitech G15 keyboard | Logitech G25 Racing Wheel | Sharp LC-46 XL1E (gaming and work)