Personally, I'd recommend going something from ULA or Granite Gear. I've always been a Granite Gear guy, owing mostly to the fact that I'm in Northern Minnesota and gotta keep it real. I got a ULA Circuit this summer, and have been very happy with it. You'd want the larger Catalyst if you went the ULA route, for the additional space and carrying capacity.
Something you didn't mention is how much volume you need. If you're using the full 5000 in^3 of that Shasta, you're going to be hard pressed to find a pack with that volume, that comfortably carries that kind of weight *and* weighs 3 lbs. The GoLite Quest and Odyssey come pretty close, though.
Don't want to be a wet blanket, but it might make a lot more sense to put the money into bringing the weight down before investing in an ultralight pack. Yes, you'd drop 3 lbs switching from the 6 lb Shasta to a Granite Gear Nimbus Ozone, but you might accomplish the same by leaving the heavy soft shell or camp shoes at home and otherwise changing nothing. Better yet, do that *and* buy a new pack- suddenly, your load is 10 lbs lighter.
I wouldn't go with the GoLite Pinnacle- with no real frame, you're not going to be happy carrying 35-40 pounds after a few miles. I've never tried any of the framed GoLite packs, seem like they scratch an itch that I don't have.