I don't know if they have improved them or not, but I would not buy another pair, especially not for real touring. Here's why:
1. The corrugated plastic stiffener board failed after about 6 months, ie, the mounting points pulled right through the plastice. Unlike better panniers, some of the fasteners were rivets that I could not figure out a way to remove without doing further damage. Luckily, Performance has an amazing return policy and refunded my money.

2. The mounting system was, IMHO, awful. It consisted of nylon straps with cinch-type adjusters to take up slack. Because the adjusters were in back, it was extremely difficult to get themtight enough. Then if they weren't tight enough or developed a little slack they could just jump right off! There was no backup to the open hooks. I suppose this could be solved with velcro straps or even a couple of strips of inner tube.

All in all, I suppose you get what you pay for, and the Epics are inexpensive. As far as size, I do seem to remember the specs saying 2800 cubic inches each. They are pretty roomy, but as Koffee says, they probably are not big enough for serious touring.
FWIW,
Raymond