i noticed the same problem got my runestone last week. Its like the arp cap doesnt make any diffrence, i tested the dps on a dummy and it was like 500 less then before.
Tonight we will go raiding some stuff i hope my dps will be better then on the dummy.
im currentle gemmed with arp gems atm i have 580 passive, wanted to try changing some arp gems to agi but im out of epic gems..
anyway i hope someone can help us out with that runestone problem.