Microstter was dealed with in the frame pacing part of the drivers, quite a bit of time ago. I had 8800gtx sli as well. sli was still in its infancy back then. Microstutter was dealt with kepler onwards. I had 580 sli as well, microstutter was much improved in fermi compared to older gen sli cards I had in the past. Judging my 10 years experience with sli with frametime graph with an 8 day game GTA V was not really the best counter to my no problems with sli. I cannot speak for xfire as I never ran it but I would probably expect more of the same.