The AVID machines are quite capable. I helped my buddy order a few parts from 8020.net to reinforce the exiting structure (extra extrusions, cross-members, gussets, etc.). I think he spent about 300-400 on parts/shipping and it made quite a bit of difference after we stiffened up the base.
I think that's where the steel welded machines have a huge advantage, but it comes at a real cost in $$$ and weight!
Remembering ...
I think just the 16 footer ... we had a custom Z axis with 16" ... plus? of travel?
Rapid traverse, with the weight of the spindle way up there was possibly overloading the 'trucks' on the linear guides. I kinda remember seeing a few small ball bearings laying around before I left ...
I think the 4x8 we extended to 12 feet had the v rollers