My wife and I opted for the Surly Racks when we started equipping for our first tour. The rack is very heavy, and mounting with the shims makes it seem harder than it needs to be (the first one took me an hour to mount, the second one about 15 minutes).
However, once the rack is mounted and bolted down (bottom and half-way up), we saw absolutely no give to the rack. And, we probably had in excess of 35 pounds hanging from or on top of the rack.
We discovered, the hard way, that the key to mounting the panniers is the Center of Gravity (CG). We found the critical element is to get the overall weight as low as possible, and as far back as possible. Making sure the panniers and cargo cannot shift or wobble will make your life much easier. Balancing the load side-to-side does not seem to be especially important, if you can keep them within a few pounds of each other.