First of all, I don't think any good can come from touching anything in the nVidia Control Panel unless you're attempting to increase image quality. So, set all of that back to default. Beyond that...

  • You're using DX11 which is going to eat through your VRAM incredibly fast and I highly doubt you could multibox 15 characters in DX11 on only 4GB of VRAM. So, switch to DX9 and that will solve that.
  • New character models eat up extra VRAM, so disable that.
  • While your Control Panel indicates you have SLI enabled, the monitoring program shows that you do not since only one GPU has any load on it. This is either a problem with the monitoring program, or... you don't actually have SLI enabled.


In addition to that, you can also drop your resolution since you're trying to play all of them at practically 1920x1080, which is suggested in the Wiki page you linked:
As a last resort, lowering the game resolution may significantly improve framerate as well.
It says "may," I say "will."