Why the game runs so well in window mode???
 

I currently use a window mode of 1334 X 768. I do get frequently 60 FPS over land and 85-130 over the sea with very little stutter! This is with all settings to HIGH apart from trees to LOW! My spec is I920 @ 4.2GHz, 6GB Ram, 480 GTX.
Will be great to get this kind of performance in full screen.

Why shouldn't it? The lower the resolution, the less pixels the system has to render, the higher the FPS.

Nothing surprising here.

Less pixels = less GPU power and VRAM required.

This game requires GPU, in both camps, GPU power and VRAM.

Through Im very happy with the performance of my GPU in this game, I can play at 1920x1200 with AAx4 well.