Originally Posted by Gavagai
I think it's likely that mid-boil additions are necessary to achieve very high levels of IBUs. Some of the iso-alpha acids imparted by bittering additions will be pulled out of solution by break material, which a later addition can help compensate for. Simply adding more hops at the beginning of the boil won't help if the wort is already saturated.
There could be some truth to that. Or perhaps, staggering the early and middle additions could be providing a smoother type of bitterness (instead of adding everything at 90/60) while helping to change the flavor profile a bit. In either case, actual IBU concentration tops off at 90-110. So there is probably something in the way of flavor happening there. This is the reason I would not be scared of a recipe that stated 200-300 theoretical IBUs; as long as an appropriate amount of an appropriate hop is used at the appropriate slot.
What I researched this initially, I compared the % of total hop use in many different clone recipes. I found that for the huge majority, the IPAs I enjoy most use a lot of hops and focus on all four additions (dryhop, flameout, early, middle -- in that order).