I ordered five pairs in the 10-11oz range that I've heard good things about, and tested them today on the treadmill at Keene Fitness. They were, in descending order of weight (top left to bottom right in the photo): Pearl Izumi Trail N2, North Face Ultra Guide, North Face Hypertrack Guide, Asics Fuji Trainer, and Asics Fuji Racer.
I wasn't fond of the feel of either of the North Face shoes, which for my stride felt very clunky in the transition strike to toe-off, and the Fuji Trainers didn't fit my heel well at all. Those three shoes were out pretty much the instant I stepped on the treadmill--they'll go back to Running Warehouse for sure.
The Trail N2s and Fuji Racers, on the other hand, felt right immediately. The Pearl Izumis are the clear winners for what I'm looking for (mid-weight, plenty of cushioning, specifically for ultras), so I'll keep those--in fact, I ran 5 miles on the trail with them later in the day; I'm committed. I'm torn on the Fuji Racers. They are very light (9.5oz in size 10.5), so they compete with the La Sportiva Helios as my fast racing shoe. I had planned to buy another pair of Helios, but the Fuji seems like more shoe (better rock plate, stiffer forefoot) for the same weight. I'll need to compare them side by side with the Helios before I decide whether or not to keep them as my lightweight racer of choice.