Do NOT use BAMT 1.5. It uses the newer 14.1 BETA drivers, which absolutely SUCK for mining. Reflash BAMT 1.3, which uses better drivers, and you'll be all set.
We lost ~100kh/s when we tried BAMT 1.5, and we immediately went back to BAMT 1.3 on all our rigs.
And yes, the optimal settings for those 280x will be using 2 threads, with a lower TC. We use SGMiner 4.1, TC 8193, 2 threads, and the zuikkis kernel, and we get anywhere from 740 to 780KH/s on our Gigabyte cards, depending on RAM type and core OC limits.
Got same result with BAMT 1.3 and 1.5. With SMOS 1.3 I got 700+ but still not on par with Windows. A dedicated Windows mining OS looks interesting.
https://litecointalk.org/index.php?topic=16545.0