Ya, the performance didn't seem hugely impacted in Windowed (fullscreen) mode. I get much lower framerates in Pandaria but that's how it's always been and I gotta deal. I have to agree it does make switching apps a lot faster and easier.
I hereby stand corrected.
** and Haleth, you're right about the simple shortcut for toggling between windowed and fullscreen. I still found it a hassle though. I guess i'm just lazy.