With regards to McLaren's poor testing in 2011: McLaren and press all said that this mainly down to the octupus exhaust system which McLaren just couldn't get their head around. The moment the octopus was replaced with the conventional exhaust system (ie. a they went a step back), they appeared to all of a sudden, gain 1s/lap, which seemed to have made them competitive.
Had they abandoned the octopus exhaust earlier (in testing), they almost certainly would've put in more respectable test times in testing.
You can't really compare BrawnGP who had a disastrous pre-season (with the exception of the testing session they attended) and then turned up to the first GP of the season and blew the competition away.
There is some speculation that the exhaust they were playing with was something that allowed them to change where the exhaust was being released on the fly to give them different data on exit location.
Not that they ever admitted it was the octopus as it was speculated.