This thread has me intrigued. I owned the Mindshift 26L back when I just had my first small Fuji setup.. as I migrated to bigger body and lenses I kept switching. Never had a complaint about the 26L other than size. Comfort and durability seemed top notch.
I currently have a Mr Jan Gear Boris IV which fits 2x Z9, 400mm f2.8 tc, 800mm f6.3, 1.4x and 2x TC with ease. however it is large and I've never flown with it. I just use it if I'm traveling by SUV. also with all that gear, it is quite heavy so I don't hike with it. I usually bring another backpack to use with either the 400mm or the 800mm once I get to the destination.
I've recently flown to Iceland twice and did not bring telephotos, but I did pay attention to their carry on specs.
Carry On - maximum size including handles/wheels - 21.6" x 15.7" x 7.8" - maximum weight 22lbs (10kg)
This would exclude the Mindshift 36L which says it is 22.4" x 12.5" x 9.1". Instead I currently have a Shimoda 35L which will fit 2x Z9, the 400 f2.8 tc, and 70-200 and measures 21.2" x 11.6" x 7.8" - perfect size for Icelandair.
Some others have alluded to flying with the Mindshift 36L and saying it is fine. I guess it all depends on how strict the airline is. I enjoy the shimoda bags because they have ICU (internal core unit) which are essentially a bag in themself, so if you were ever flying and told it was too big - you could just pull the ICU, keep it with you, and put the bag in storage. That being said, my 400 2.8 does not fit with a body attached, and there's no way it would fit without the hood reversed... and certainly no way to fit a 400 + 180-600, as Bruce has done with the mindshift 36l.
food for thought. I love these bag discussions. have many pictures saved of different setups