my mate and i (me 20 WF Barb, he 20 WF Fighter- both THF) decided to put this entire speculation to rest. Us two, and our WF Sorc friend zoned into TOD (just for the sake of it being a combat zone) and had a bit of a swing contest. Both of us recording with fraps as well as watching each other, we find that with absolutely no buffs he and I swing at the exact same speed (we both have our respective capstones)
HOWEVER--------------
When our sorc buddy casts Haste upon us- my Fighter friend very clearly swings faster. We are in the process of counting the exact swings but it is visable and clearly inarguable. Once we finish our counts and youtube finishes its upload i will post our videos.
Basically, i am totally and 100% positive that this seemingly impressive Barb Capstone Speed Increase is bunk as it does NOT stack with Haste. It does, however, exist. Capstone Barbarians are attacking as fast as Capstone Fighters if both are totally unbuffed. The Barbarian boost does not stack with haste, the fighter one does.
Draw your own conclusions on if this is a significant game changer.