Mean Sea Level? Never used that term before. Anyway....
Since the car is turbo, altitude has less affect but it still must be meaningful. I really dont know. That is why I do my own tests, and also because it is fun! My air is very dry like 10% humidity which makes heat transfer more difficult and should take away ponies but the air temperature was in the 60sF so it wasnt hot. There is nothing better than cool dense ocean air. For the quarter mile I would assume that I would loose 2-4 tenths of a second due to the air.
What I have been surprised with is that the car didnt obviously pull timing or boost after 30 minutes at full song on a road course. Even a ZR1 will get too hot after several hot laps (I am told).
Another reason that adding power to the 12C will reduce the safety headroom. More power, more heat and so far the 12C manages heat quite well. It does put the water(?) for the intercoolers in the prime front airstream. I will reserve final judgement until this summer when it is 105F ambient.