dunno if this is quite what you wanted Chris, but in this video 1318 litteraly drives circles around the defending robot in green, 3251. Although without any other drive-train details it is difficult to tell what really happened, this is an instance where i would say a mechanum robot definitely out maneuvered at least one non-holonomic defender with ease. Although this video says little about a mechanum driving circles around a defender like 973 (substantially more tenacious than the defenders in the video) I think that it shows the capability of a mechanum drive-train to use the ability to strafe to glide between defenders in a way that would be difficult for a tank drive to do.
http://www.youtube.com/watch?v=lwv9hZgw9r0