Will reducing the number of or disabling spaces improve performance?
I checked it out in Instruments and I could barely notice a different in the activity monitor instrument, probably because I am looking at CPU and RAM bound items. 
There obviously is going to be a higher load on the GPU than if there were no spaces at all, but I doubt its drawing everything in all spaces at all times, and like Martin said, it all depends.

The real possibility is that with so much extra space that you run many other programs which in one space you would find cluttered and would have automatically closed them due to aesthetic reasons.