Hey Erik-Jan,

From the Mix Option Window, you can make a mix a range of sizes up to 1920x1200. Using Catalyst, this is the highest output per mix you can do.

If however, you are not using mixes but use full screen outputs, you are not limited and can output files up to the max resolution of your display.

I have Apple 30" displays and they work with full screen output (sort of).

The bottom line is that the mixes will do 1920x1200 or 1920x1080.

It's the content that is important. If your content is not rendered at the resolution of your screen then using a higher screen resolution is useless.

In your case, using 1080 should be able to output 1080.
I have in a test environment on a QuatroFX 5600 with 2 TrippleHead2Go boxes (in dual head mode) been able to run 4 1920x1080 mixes to 4 plasma screens.