Team 1310 used 8" mecanums in 2010, and used them to go over the bump.
I won't claim that the forces are the same as going over this year's barrier but at least I can share some real experiences.
The wheels lasted through 2 regionals. We had to replace 2-3 broken rollers I believe, but broken rollers didn't cost us any matches IMO. You can lose one roller, and it doesn't impact you that much during a match. The bigger problem is that the driveshafts eventually bent a bit. They never failed completely, and in fact we never did replace them, because it was so difficult to get the wheels off the bent shafts.
We had to hit the bump pretty fast in order to carry enough momentum to get over the bump, so they took a pretty good beating.
However, 1310 employed a very very simple suspension that probably save both the wheels and driveshafts from a lot of damage. Some would argue that you should have a suspension when doing mecanum regardless, to ensure all 4 wheels are touching the ground as much as possible.
If I were to consider mecanum for this year with the intention of going over the barrier, I doubt I would do it unless I also put serious resources into a suspension system.
A few practice / build season video from 1310 in 2010:
http://www.youtube.com/watch?v=PneD_...0&feature=plcp