The 186.16 drivers do not work for me, even with the newer GXM. Had to roll back to the 182.50 drivers.
Nor me and most people. The 186.24 however do, I posted a thread about it. You will likely have to run at 55/56Hz to get it past the custom res test.
Great review btw, very interested in these 1792MB cards. I second the 2GB 285 test, that's a monster of a single GPU card.
The way the performance flattens at 5040x1050 on both the 295 and 275 1792MB suggests to me either the drivers are not optimised for this res or there is an internal resolution "safe zone" limit, like back on the 6 series which occured around 1920x1200, IRC performance plummeted over 1600x1200 no matter how much video memory you through at it and it was only rectified with new hardware.
Incidentially, which App did you use for the graphs, they have a very cool "paper" look?