Your machine is looking fantastic !!
We did a similar dual axis Z a few years back and it turned out great , we estimated it could lift at least 100lbs, with 270oz motors, and if I can remember correctly the screws were a 5TP
You should feel confident about your design it should have no issues lifting the Z gantry with two motors,
here’s some pics buried somewhere in these links
http://www.rockcliffmachine.com/forum/showthread.php?t=111
http://forum.canadianwoodworking.com/showthread.php?28369-Home-Made-Aluminum-CNC-Router
the customer on this machine did eventually use some counter weights, but only because the acme nuts were wearing a little more then normal
with the additional load weight,
I would not expect any issues with your ball screws
Your going to love the rotating nut there great, takes a bit more money and time but great,
Just have a question for you, on your rotating nut, did you have to center the nut in the bearing, I only ask because every rotating nut I have done in the past the center line of the ball screw has never been true to the flange, and I have always had to leave space to adjust the rotating center line?