I think the motor on the z axis is going to be powerful enough for the 27 lbs servo motor (especially once I loose the BLDC driver and control panel). In finding used motors I bought very powerful ones for fairly cheap (I got three of these motors for about 250 each: https://www.teknic.com/model-info/CPM-SDHP-3421P-ELN/). Got lucky there. Right now with everything as it is the z axis motor on rapid moves is using about 30% of available power according to the clearpath software. If the motor isn't up to the task, I could replace it with a more powerful one without too much trouble. I would prefer not to go counterbalancing just for space behind the mill, but could if needed.
(edit: after looking at the available motors I think I would actually just build a belt drive reduction if I needed more torque. The motor I have can spin at 3k rpm and I am using nowhere near that. I could belt reduce it to 3:1 or even 4:1 and still get the rapids speeds I have now (3000 mm/min - 118 ipm)
Doing more and more research into this I think I am going to go the servo route. It seems that it will be about the same amount of money, slightly more constant power, way more peak power, all in a smaller, lighter form factor.
I am still waiting to hear back from Teknic (most of this research happened over the weekend), but if I do decide to go this route I will be sure to document it well and report back!