I wouldn't judge results of AMD's SLI performance for dual 9800gtx or GTX280s using a 790FX chipset. I am familiar with the hack you are referring to, but if memory serves me correctly, the hack was not that great. Yes, there was an SLI speedup, as you pointed out, but I do not believe it was mature enough to give similar results as say an SLI speed up on a 780a or 980a (or even p55) chipsets. That is likely why you noticed a much bigger gain by adding your second nVidia card to the Intel platform, than to your AMD. Had you been using an Nvidia Phenom chipset, you likely would've had a different experience.