I managed to get my Rogue the rest of the way through Outland yesterday and have him now safely parked in Warsong Hold in Borean Tundra - all ready to get eaten by big undead spiders out in the quarry!
So all four of this last set of toons have now made it to Northrend and are ready for the final push. I did pick up dual spec on the rogue so that I could try an assassination / mutilate build for running instances. I wasn't finding it particularly effective for leveling, and actually wasn't noticing any difference in my dps in instances either. The primary difference for leveling is you lose some of the AoE capabilities that you have with combat in both Blade Fury and Killing Spree. It is also to your advantage to start all of your attacks out of stealth to get Overkill going and that just takes a little longer. While it doesn't feel quite as rogue-ish, combat has a lot more ability to just walk up and start smacking guys around. So I guess that's 1000g down the tubes for nothing!
I'll be picking up the priest starting today and hope to get him a few more levels before switching over to the mage next week. I've still been debating just doing them one at a time from here on out, but then there's part of me that thinks it would be pretty cool to have four toons all hit 80 within a week or so of each other, so I'll probably stick with the current rotation for now.