I really like the idea of a single glass above the negative, I tried this in the past with mixed results.
As others point out I use mainly condenser enlarger with a oversize 250w bulb which does push out a fair amount of heat. I find that I can close down 2 stops and print 16x20 fibres at 15 seconds.
With split printing and complicated burn patterns a shorter print time is really nice with the hotter bulb.
If I could make a single glass work well in my enlargers I would switch to that method in a heartbeat.