I've got 26x1.50 Nimbus 'dillos on my Hardrock Sport, and it flies. Honestly, speed wise, I can't tell a huge difference on flats. Downhills, I run out of gears compared to my (now former) Trek FX so that limits top end to the lower 30's. Uphills, the suspension slows it down. Overall, though, I average around 15mph for a week's riding, which is about .5 mph less than what I'd average on my 700x32c tired Trek.
Now, that being said, when I lay the hammer down (so to speak) I bet if I didn't have the suspension fork I'd notice no difference. I've passed roadies and blasted beyond 30mph on straightaways, while also relaxing on the way in today and enjoying the nice smooth ride when compared to the jarring 110psi ride of the 700c's on the Trek FX.
Overall? Love it. I'm going to pick up a Surly Long Haul Trucker soon, and will give another comparison then, but I really love the way my 26's ride.