This may not be an issue with the game. A blue flicker usually indicates either a bad SLI bridge, or an improperly seated SLI bridge. Try the following:

1. Reseat both connections on the SLI bridge and retry.
2. If you still get flicker? Flip the SLI bridge around and try the game again
3. Still get flicker? Replace the SLI bridge with a new one

I got this in other games and flipping the SLI bridge around worked for me. Hope this helps

EDIT: I hope this doesn't have to be stated, but please make sure you shut down your PC before attempting any of this.

Arma 3 is a fairly CPU intensive game and right now it seems it is only really using 1 core. Multithreaded support doesn't seem to have been added or enabled so far. I'm running 2x GTX580 in SLI with i7 960 OC to 4.0GHz, 8GB of DDR3-2000 CL7 ram, 3GB total GDDR5 and on moderate-high settings I'm getting about 25fps in towns, about 55 FPS in wilderness and 90 or more FPS staring at the ground. I'd expect more FPS once proper multithreaded support is added and/or enabled.

So although you are correct that Post Processing does eat a fair bit of FPS, I can't help but think that most people's FPS is being affected much more by a lack of multithreaded support.