Cool things happened. First of all I fixed all my fullscreen issues (there were a lot !) and the game can now be played at any resolution. I also made sure VSync was enabled in fullscreen only if specifically asked. And that's where the fun begins... While keeping the same resolution as in non-fullscreen mode (1024x768x32), I switched to fullscreen and performance jumped from 220 FPS to... 405 FPS !! And that's with mid-res realistic tiles ! Simple cube tiles reach over 700 FPS, can you imagine that ? Of course these numbers are only theoretical since our eye can't distinguish more than 80-100 frames a second...
But it's a 2x jump in speed. I tested the code on my GF3 machine and unfortunately there is no improvement at all. ATI confirmed that this is a known issue with their drivers : they're optimized for fullscreen modes only, not windowed modes (I verified this on the DX8.1 SDK samples). I'm a bit surprised because KMJ 18.75 runs at 200 FPS both in fullscreen and windowed modes... Only the upcoming version reaches 400+ FPS on Radeon 9700s. Well that's good news anyway, isn't it ? I don't know if ATI will ever optimize their drivers (they told me it was unlikely because most DX games run fullscreen, and I guess I can understand their point), but if you have a Radeon I'm sure you'll be waiting for the new release even more impatiently than everyone else !