I find knobs are only slower at high speeds on pavement, where they buzz. On gravel, rolling resistance is more related to casing flexibility. There are all sorts of test with some pretty knobby tires rolling great. I do find them more stable descending any sort of loose gravel road. And having ridden that gnarly section in Montana in dry conditions just last week, essential if you want to try and ride it.
On the other hand, those tires which feature a big slab of rubber with a modest thread cut in, such as the Schwalbe Marathons will wear forever but ride like a dead fish. I have a 26 x1.75 Marathon and find it amazingly heavy and slow.
Here's a good thread from bikepacker.net on what tires people use for the Great Divide. The WTB nanaraptor and the Geax Saguaro seem popular. http://www.bikepacking.net/forum/index.php/topic,1924.100.html
Here's some test data. The Geax looks good, and the Continental X king looks phenominal.