Did you check your pre-rendered frames setting? Did you try low-latency vsync mode? I'm not trying to say you are wrong here, but I believe there are already alternatives in your case.
As for the math, we can assume that your monitor is running at 60hz. We need to assume (for simplicity) that the CPU and GPU are running out of phase (as in vsync off with no synchronisation). A frame renders roughly every 16.6ms with whatever is currently drawn to the buffer. With a 120fps limit (which actually ends up slightly higher due to internal latency optimisations), in the worst case, the last frame drawn was drawn 8.4ms earlier. In the best case it is drawn 0.0001ms earlier. The average between these is 4.2ms, so I guess the different is more like 4.175ms. The figure I gave before was a ballpark -- the important part is it is already low enough that it shouldn't matter.
More than providing a higher fps limit, I think you guys would enjoy a hardware mouse cursor. That way you are not getting any frame-induced input latency. THAT SAID, low latency vsync (for the nth time) should be about ON PAR with this, so I highly recommend you try it. If it works well, I can merge it into the next build without much of an issue.
For the energy conscious, use 120fps. For those wanting no sacrifices, use unlimited (preferably the new during-gameplay-only option). If you are still worried about your card drawing power, I highly suggest investing in a greener card. Other games will be drawing just as much if not more power, too.
