An interesting exercise I just run was to calculate the average QB rating for a few top QBs for plays when they have been pressured (all the stats are from profootballfocus.com). Now I know that this doesn't take into account how much they have been pressured (for example Rivers has been pressured over 80 times this year) and maybe I will do some more calculations in the near future. None the less I think it is interesting to compare. I have ranked them below in order of pressured QB rating with their non pressured QB rating next to this number.
QB rating (pressured) QB rating (non pressured)
Manning 86.2 110
Romo 79.8 102.1
Brady 79.5 109.2
Rodgers 68.6 97.7
Rivers 67.9 105.0
Brees 67.3 103.7
From this table we can see that Manning seems to be way ahead for performing under pressure but quite surprising is that Romo is second. Also there seems to be a big difference between Rivers' pressured and non pressured ratings which may be a factor of how many times he has been pressured. In the near future I may do some more calculations to take these into account.