I'm gettin 1450 h/s with 4x 280x  (so 362 h/s per card) I tried everything and I can't get it to 450 h/s like anyone else.
Win 8.1 64bit, stock clocks, stock voltage, 13.12 driver ...

I noticed in AIDA64 that only 2 GB of GPU ram is in usage per card when mining. Is mybe that the issue?