Not claiming expertise here but the max for the heavier bullet would have to be less with the same powder. I've never used 2520 but it's that way with the powders I have used.
Does the 80gr have a longer overall length than the 75gr?
Also not sure if your set on the A-Max but here is some data for the 80gr SMK.
http://www.whiteoakprecision.com/info-reloading.htm
Edit: Just found this also, it has a few with the 75gr A-Max. Reloading data half way down.
http://www.njhighpower.com/cart_tech.htm