You can model a mecanum wheel as a regular wheel of the same diameter and tread material but with 71% the coefficient of friction (assuming 45 degree, friction-free rollers. In actuality, it is >71% for F/B and <71% for L/R due to friction.).
When you aren't slipping the wheel, you get the same speed out of a mecanum wheel as you would with a regular wheel of the same diameter. It is a common misconception (and one I had believed until recently) that a mecanum wheel is necessarily slower than any other wheel.
See here for the math:
http://www.chiefdelphi.com/media/papers/2390