Weight would still be an issue, just for the Y-axis not X, plus Z should go faster.

Presumably before Jazz 'went vertical' he had a decent safety margin in his X-axis feedrates so he just used up some of this. Just as an example, if your X-axis ballscrews are 10mm pitch and the mass of the gantry is 50kg, then making it vertical will require 50g=490N more force to lift it, which corresponds to:
T=FLe/(2pi)=490*0.01*0.9/(2*3.142)=0.7Nm extra torque required.