The stock tyres look similar to what came on my specialized hardrock - they don't look too knobbly. The reason I changed them was not for rolling, but for puncture resistance. I can't say I noticed much difference in speed, but I guess the marathons are heavier, so it might have cancelled out the benefit of the tread pattern.
I do notice when riding with a friend on nobblier tyres than mine, I think racing Ralph, my bike is faster than his on the downhills without pedalling, so clearly it makes some difference.
Just saying that I might not bother.