Really dont see how removing the engine cover is going to do anything for reducing engine bay, or manifold temp's as you havent taken any of the contributing factors to engine bay heat out of the equation and the manifold is still getting both radiant heat from the sun on the bonnet (during the day of course) and getting soak back from the rising heat in the engine bay either way.
Somebody on LS1.com did a test with cover on, cover off etc and the temperature was pretty much identical from recollection, and certainly not reduced in any great way. It's more of a cosmetic thing, although, you could be like the guy in the VZ section who painted his engine cover in heat proof paint and claimed he got extra KW out of it, BigDaddyCool i think it was?
If you're seriously serious about reducing engine bay temperatures then the first step would have to be HT Coating the headers. Then if you want to get extra extra serious,you could try relocating the heater hoses to remove their heat from the equation etc. etc.
Bottom line. Performance gains = minimal
Aesthetic gain = personal preference