I now have two lathes using Yaskawa servos.
I'm using:
SGMJV-04ADA21 motors (400W. 1.27Nm). They seem to outperform their ratings, plenty of torque even when going really fast.
SGDV2R8A01A drives.
The tuning software is super friendly. The documentation is 100%
For the first lathe I converted, I initially had steppers and then a forum member gifted me some old surplus Yaskawa drives so I replaced the steppers. Night and day difference, so much so that second time around I just starting searching on eBay for the same Yaskawa motors and drives because I'm so impressed with them.
For my mill I'm using StepperOnline ac servos and they are working great (can easily lift the knee at > 100IPM). The tuning software and support however are quite poor.
I think once you have had a taste of servo performance, you never want to use a stepper again.
Tons of Yaskawa servos and drives on eBay, surplus typically about $100, new not much more. IMHO, the differential price to upgrade from steppers is well worth it for the performance.