My new 58mm carbon clinchers haven't done much to increase my average soloing speed, but that's okay since I didn't expect them too.
My goal was to have them give me a 1-2% increase in aero advantage in a group. This 1-2% will help me stay with the group longer and in turn give me a much better workout, which in turn will help me gain more fitness, which in turn will make me faster. Sheesh.

Sidenote: I did notice I was able to hold my speed at lot easier on Saturday with my 58mm wheels vs my 30mm wheels. Usually when I fall off the back I slow down to 18-20. On Saturday I was able to hold 21-22 mph. I can't say for certain the 58mm wheels are to thank for this, they probably only helped a small amount.

As AdelaaR said above, they certainly bring a nice placebo effect.