Yep it's windowed and the size is maximised to cover the entire monitors.The difference in performance is really really big.
I'm suspecting it's because of the crossfire setup(2 x ATI 7950). It seems when I run Sins full screen both gpu's are working. On average something like 70% and 50%. When I run windowed only 1 gpu is used which is running at something like 30%.
I'm going to disable 1 gpu tomorrow and try to run it fullscreen and see how it performs then. I'll post my findings tomorrow. Does anyone else have problems with multi gpu's?