I had replaced the heat sink, re-applied a sane amount of Arctic Silver thermal paste, and used Lobotomo Fan Control daemon to monitor my system's temperature. You can conduct your own testing of course. Moreover, the newer models have far more efficient blade fans and would suffer even less from the effects of heat (additionally, the new Intel chipsets run cooler than their predecessors). I ran the system for well over a year with the clamshell open, and then closed, and saw no different in CPU/CPU temps (nor any variability in the other heat sensors) as a result. Personally, I have a 2006 MBP that was eventually relegated to being a desktop. Some claim that keeping the lid open allows the chassis to cool down, but these claims have never had any real data behind them (simply anecdotal evidence and unsubstantiated rumour). Ambient air is never in short supply and the 'exhaust' (where the hot air picked up by the blade fan(s) are expelled) is located at the back of the unit (the black stripe on the newer models and the grey stripe on the first generation machines).
Having the lid closed or open does not impact the airflow of your MacBook Pro (MBP).
