It doesn't seem to make much of an impact that I can see. It may on some systems.
I picked up a copy of
http://www.wowinterface.com/downloads/info13555-InterfaceUsage.html which was brought to my attention by a nUI used (thank you Corey) and that confirmed what I pretty much already knew -- nUI_UnitAura and nUI_UnitSpec are hammering the system in raids with nUI_UnitAura being the worst of the two. So, I'm going to try and get nUI_UnitAura rewritten this weekend and, if possible, will tackle nUI_UnitSpec as well. Between the two of them, I would expect to see a non-trivial improvement in frame rates in nUI, especially in 25 and 40 man raids.