i cant seem to figure out why performance would be hindered while the computers closed? you would think it would do better because its not worried about pushing its internal display?
well, my thoughts is when i am playing diablo 3 with the lid closed, it exceeds a temperature where the cpu throttles to lower the temps... with a throttled cpu you're gaming at roughly 15-20fps, a pretty unpleasant way to game. anyways, i'd recommend just trying it with your setup, mine has two thunderbolt displays with increase idle temps of the mbp by about 10c per monitor, with both i idle at roughly 60c to begin with, maybe with one standard 1080p monitor it will be playable with lid closed.... i can try with my hdtv at some point...
anyways, it may vary with game some, but i can tell you there's no performance impact with diablo 3 using the internal display & 2 tbs, in fact i get the same performance as i do with just playing on my laptop's internal display without the thunderbolt displays plugged in period.
it's all to do with the game's settings, i can't play diablo 3 greater than 1080p on my thunderbolt displays, and if in windowed mode i have to leave it at the window size of 1080p, maximizing the window is like playing at 2560x1440p... aside from that, i turn off shadows and lower ground clutter, game plays at 42-45fps, same as if just using laptop display w/ same settings, though i prefer playing w/ vsync on which pretty much locks it at 30fps, this game plays real smooth at 30fps.
anyways, i wouldn't worry about hurting the computer testing out what works for you and your setup, i have two 2011 macbook pros in the house, and have used both in clamshell pretty hardcore for the last half a year. it's not gonna harm the macbook, though may screw with your gaming performance... if you do decide you want a lid open "clamshell" you can do that with a terminal command in lion, just google it.
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Like another poster said, closing the lid reduces wifi performance, and with Diablo 3 being in a sense online, it would make your game lagg.
i don't think that is it at all, i have ethernet cable hooked into my TB displays, i think the cpu is throttling from temps in clamshell, anyways, i'm sticking to my guns to just test it with your set up and see what works for you, but lid closed/open a variable to tweak w/ if your having performance issues... it definitely solved mine.