Gary,
I totally agree with all you have said but I thought that I was following in well trodden paths in choosing steppers that others had pointed out as being strong enough. The mistake I made was in not doing as they had done in dropping the 2 to 1 gearing by using toothed belts etc nad going with direct drive this coupled with the coarser pitch of the ball screws I think has left the z-axis under powered.
I have also just found references to the fitting of dampers to the steppers to alleviate resonance, not that I know what to look for if I have got it!!
On the web the videos supposedly showing stepper resonance and displaying missing steps for the life of me I couldnt hear any difference.

Peter