I have the topeak super tourist DX tubular rack and use it regularly with their MTX trunk bag. I have no problem attaching full panniers on those days when I use them (fold up the baby zip out panniers, put my real panniers on with the trunk bag. But note on my version of the Topeak rack, the top pannier hook bar is a little lower than the shelf. If your rack is NOT that kind, and they make several models, you might have more of a nuisance where either you have to mount the panniers first and then slide on the computer bag, or maybe even there's some clearance issues with sliding on the computer bag.
See http://www.topeak.com/products/Racks...tDXTubularRack and compare to some of their others.

As far as that computer bag, I looked at it in the store and wasn't particularly impressed with the padding. I'm still a bit of a nervous nellie that worries about jarring my laptop around with shock transmitted through the rack, even though hard drives are getting better. On the infrequent times I need to carry one, I put it in a sleeve and carry it (alone) in a messenger bag so my love handles add extra cushioning. Or put it into a pannier with a towel or some clothing between it and the rack. I put all the peripherals and crap into the (other) pannier.

This bag didn't look any worse than a non-computer specific bag for its padding, but not much better. And given the two REI reviewers comments about the design squishing the laptop closed from that road noise and weight, I'm not taking it for a test drive.