tbh i think it would run cooler because of where the vents are on the back of the machine. when u open the lid the bottom of the screen blocks the vents but when closed they dont......
Should be fine. In iBooks, the keyboard was used as part of the ventilation system, so when people hacked the firmware to have it run in clamshell, overheating was a problem. The MacBook was redesigned with a new ventilation system, which no longer requires venting through the keyboard (keyboard redesign could not allow for ventilation anyway). Running a MacBook in clamshell should not in any way result in higher temperature.