So what is the shear strength of a 3/8" grade 8 bolt across the thread section? To me, that has to be part of the discussion when comparing how 6" will be used versus 8".
I've always maintained that the entire threaded section must be fully inserted into solid wood because it's the integrity and strength of solid wood (not bark) that supports the bolt's shear strength. The weakest link would be a portion of the threads that might me not inserted into solid wood.
So getting back to the drill part of the equation when boring a thick barked tree...Will the bit drill deep enough to penetrate the thick bark and still bore deep enough so that the bolt's threads are completely encase within solid wood? I think the point of using an 8" bolt on such a tree is the user's tolerance of how much bolt is available for hand and foot support. If the bark is ~2" thick, then it essentially reduces the usable portion of the bolt by 2" when compared to a tree with thin bark.
We can't discount the leverage factor of the 8" bolt. An 8" bolt will tend to bend easier than a 6" but, if there is no thread exposure (the unsupported thread section) is not a factor then will the leverage bend the 8"? I think that's an answer that each individual will have to decide for themselves.
And regardless of bolt length...Keeping the bore hole nearly level will reduce leverage.