I don't think you'll see much speed difference in going to 650x23C, even if you now have a 26x2.0 slick on there. I've run a couple different tire widths on my MTB and I don't see a perceptible speed difference between 26x1.4 slick and 26x1.0 slick. I run 26x1.4 most of the time on the commuter wheels because I can run them at lower pressure for more comfort. Many current MTB rims are narrow enough to accept a 26x1.0 so try that if you haven't already.