I believe the gold can that sits on top of the pump is a barometric pressure compensator. I'm not exactly sure how it works, but would expect it has some kind of air bladder attached to an adjuster that richens/leans the rack as pressure changes.
So at least the MFI system has air pressure compensation built into it that might give you some shot at tuning to your conditions.
Edited to add: see this thread for more info:
http://forums.pelicanparts.com/porsche-911-technical-forum/544158-mfi-barometric-cell.html