Oh sorry, I hadn't actually tried it myself. I just remembered coming across it when I was looking for the option after noticing vertical sync only really working in windowed mode. But then I read a lot of stuff about windowed mode actually causing lower performance so decided to stick with full screen.
If you're only reason for this is to switch between windows, can I suggest using alt+enter to make the game windowed mode before alt+tabbing (if you're not already aware of that). When alt+tabbing from full screen, the game often crashes. Alt+enter beforehand to make it windowed mode, then alt+tabbing, I can't recall a single time I've crashed. Just throwing it out there, even though it looks like from your previous posts its more about a matter of speed... but shit, what's half a second extra eh? The universe is infinite.