The 65mm original mount does place the router/spindle under the Z-stepper. I had a 800w 65mm spindle installed, and it had to fit under the Z-stepper. It fit, but it was tight.
The 80mm mount has more clearance, allowing a spindle to go on the outside of the Z-stepper. So, no more restrictions on height! Its a very tight fit - only .1" of clearance in my estimation - but it does clear it.