I would have to think that 2mm would be very hard to tell when riding. It's almost like asking someone if they can feel a 1 horsepower improvement in their car. It's all in the mind.
I do have the Carbon SL 142+ with the DT swiss rear hub and it's a great wheel for the price. My other wheel set is the alloy controls with a 142 CK rear hub. I can't feel the 2mm difference when I switch between the two. My rear derailleur, however, can. I have to fine tune it each time I change wheelsets.