I just built a Surly Crosscheck and I'm presently running 6cm of spacers plus the brake cable hanger.
I'm sorry, but it feels as stiff as anything. Also, when I installed the fork I cut about a quarter inch off the top of the steer tube. Based on how long it took me to cut it, I think that a steel steer tube is going to be able to hand about as many spacers as you can fit.
I also was in a shop where they had a number of Long Haul Truckers with equally tall stacks. And I'm sure the Gunnar frame is a lot like the Surly in terms of bombproofness.