A 400W servo (rated at 1.2Nm @ 3000rpm) is plenty enough. 200W is also probably enough.
I will be using a Yaskawa 400W servo with brake for a 60kg Z axis (moving part only), driven by a 5mm pitch ballscrew @ 1g accel and 20m/min.
The servo has a peak torque of 4Nm, and moving this axis will only require about 2Nm peak.