I've always been under the impression you get double the normal rage gained from damage taken. It's often overlooked, there's no reason not to pop it before switching to Defensive Stance to Bloodrage before a pull.
Well, my gear is beginning to bite into my rage generation, and I was playing around with slotting zerker rage into my cycles.
From what little testing I have done, the increase is not 200% more like 150% or even less. When I tried in uldaman I gained 200% rage (1 became 2, 2 became 4). When I tried in heroic underbog it was more around the 150% mark (10 became 13-15).
Hmm annoying, why hasnt someone else done this work, it seems a really obvious question to me...........
I have no hard data. I have no logs. I did however run a series of tests with a mage friend of mine and our math indicated it's approximately a 2.6x increase.
Incoming rage generation is a somewhat mysterious mechanic despite the blizzard-posted-formula. Actual rage generated gets rounded off when the damage dealt to you results in a decimal point...one would assume that the number is rounded off to the nearest whole number, but that doesn't seem to be exactly the case. If a single attack fails to deal enough damage to grant you 1 rage, subsequent attacks TOTALLING enough damage to grant you 1 rage will eventually award you the rage. So rather than each attack rounding off to 0, somewhere the game is keeping track of the damage dealt to you (this internal tally seems to reset when you drop combat) and awarding you rage when you reach whole numbers.