Richard, I have noticed marginal performance differences between single and dual monitor setups. But they are really marginal at most. I have three monitors attached, but for simple testing I often only use two or even one. The third one is a full screen diplay of source or program monitor and often left powered off. But there seems to be a very, very slight performance difference between using PR maximized or stretched across two monitors. Not enough to give different benchmark results, due to the inaccuracy of the Windows timestamps, but I have the feeling that maximized on a single monitor can sometimes make a difference of around one second in our testing.
In practice you will be hard pressed to notice any difference at all.