I believe the point of contention would be if someone asks "should I wait for a motherboard with AGP8X to come out several months from now and pay through the nose for it, or go ahead and build my system now?", then one might wish to know whether 8X is really going to make that big a difference.
I haven't seen any comparisons between 4X and 2X mode in a long while. Only saw them back when 4X was just starting to come out, and even then 2X didn't have that big an advantage over non-AGP video. It'd be nice to see a new benchmark set based on current games, to see whether 4X is really helping all that much (with sidebanding on and off, fast-write on and off, etc).