First time I heard about the 4-8" myth..
Naturally you will lose efficiency when you brew high gravity imperial strength beers. Most cases due to "under" sparging due to fixed kettle size vs grain/total liquor ratio. You may consider continue sparging and running off to a separate kettle, boil it down and add it back to the main kettle to compensate for the "loss" in efficiency or use more grains.
My mash efficiency starts to suffer if I brew beers above 1.070 because I have to stop running off at 29 litres (30L stock pot), leaving behind sugar in the grain bed.
The only reason I can think of the shape affecting efficiency is in the lautering and running off than the actual mash itself. I run off slowly because i'm basically drawing from one end of the cooler box with a bazooka. If I run off too fast, then the wort will just fine the way of the least resistance and there will be alot of sugars in the grain bed left un-rinsed. So consider the shape of the mashtun and the speed you are running off, the channelling of the wort, and if you are drawing from a specific point in the mashtun or from the centre.. etc. No such problems if you batch sparge tho!