Just an FYI, I just remeasured, and I got 78mm in the rear. So a 2.6 in tire would be great, in my opinion. That leaves nearly a quarter inch on each side clearance. I have about 1" clearance on both of the bridges with 2.4" (true 2.1-2.2") continental mountain kings, so a 2.6" tire would leave around 0.5" clearance at the bridges. About the minimum I would want to run, unless I knew there was going to be bad, sticky mud.
One thing you might want to consider is front dérailleur clearance.. with my 2.4 racing Ralph's there is just a few millimeters if that.. it's the absolute max I could go..