One of the things I noticed on MTB's- Is that rounder tyres are faster on Hard Pack. Not so much the hard pack but the South downs way (a 100 mile trail along the top of the downs) has a varied surface from mud to loose scree- to hard pack soil and plenty of Grass. When you hit that grass- with a flat Knobbly tyre it gets to be hard work. The exposed side knobbles grip the grass and cause drag. With the rounded tyre- you get grip on all surfaces except slippery rock or Tree roots and more important- in mud and scree you get below the gloop to the hard surface below and then get grip. But then I normally use 1.8 to 1.95 tyres and only use the 2.1's in the summer when the ground is rock hard and I need some suspension.

I only run one width of rim on the road bikes and only use 23's so can't comment. But run 20psi lower that normal and drag goes up but grip with the flatter tread is increased. Never noticed any difference in handling though till the pressure goes way down.