just tested up to 11 screens.
2 matrox triple head on my second graphic card (in this case it was a 9600 gso) - setup with matrox on 2 different graphic cards won't work (or maybe i did'nt got it... restarted many times

)
all screens/beamers @ 1024x768, one output and gui screen 1920x1080.
resolume was set to 4096x1536 (8x 1024x768), running 3-4 layers @ 25fps (footage resolution was 1080p 10 second loops or flash footage). with 2 layers about 40 fps (did'nt tried to deactivate global effects&blendmodes).
ir mapio also did it's job as comp-effect, no performance problems.
then tried r4 arena beta 1, just tried short to set comp resolution @ 8192x768. output got jerky and after advanced setup the gui disappeared
so my test so far, config see my sig
as my conclusion i think i'm right to say that the memory interface on the graphic card makes very big performance differences (now in high resolution setups with 6 screens upwards

)
the ati 6970 with 6 outputs did'nt performed ad good as the older "slower" gtx285 with extra graphic cards for extra outputs (incl. triple heads).
ati has 256bit
nvidia gtx280/285 has 512 bit
a gtx580 "only" has 384 bit, the upcoming gtx690 has again 512bit.
only "problem" with higher memory bandwith is the higher power consumption!