Very little. I find my efficiency increases slightly as my ratio increases. Maybe 2-3 % as a WAG.
Edit: To clarify, as you run more water through the mash, you lauter efficiency will increase as Euge said. That could be a big increase depending on how much water you use. Also, though, I find my conversion efficiency to increase also as I mash at higher ratios. That increase, while significant, is smaller. But the best thing to do is for you to play around with ratios as much as your system will allow and see for yourself.