I was a little leery of posting this here, but from what I can gather this seems on the up-and-up. It's posted on the SWTOR forums, and there hasn't been any moderator action... which could mean that they're ignoring it, don't know about it, or are supporting it only by winking at it.
Regardless, having Steam Overlay functionality - especially when it takes 15-30 seconds alt-tabbing in and out of the program - is just plain awesome. I am going to set this up and see if I can get it to work for me in a little bit; I'll probably be on Vent when I do so as well.
SWTOR Forum Thread: http://www.swtor.com/community/showthread.php?t=104668
I've been running ToR in windowed mode, then maximizing the size (options in the preference menu). I have no delay when switching between windows and don't seem to have any issues while running the game, all settings are on max.
I don't know much about what a steam overlay actually is, but it sounds like it would be the same thing I am doing now, only that there would be no windows program bar shown on the bottom right?
Steam Overlay is what you get when you open a game in Steam and then hit Shift-Tab; that whole web browser, Steam friends list, F12 screenshot functionality, and so on. For you, this probably wouldn't mean much, and that's fine. For me, well, I don't like playing games in windowed mode - not sure why, just don't. This is very useful, and I have confirmed that it does work. It's an option, is all I'm saying.