Read through your blog, nice documentation of everything! At the rate you are going you are going to pass me up! Looking forward to following your build, hopefully we can share some ideas. You mention that the stand is better stationary than mobile, did you find that it lost rigidity once you put those casters on? I would have thought with the rubber at the bottom when the wheels were not engaged that it might actually provide better vibration dampening than the steel on it's own.
I am still debating what I plan to do for a stand. For now my plan is to do the X and Y on the provided stand, and decide before I put the Z axis back on the base. If I use the included base, I will either do casters like you did, or I may use hockey pucks as I really don't need the mobility, or don't plan to, although it would certainly be nice.