I had a look at my excel file, coming back to this servo, the 54k is not voltage limited at 48V, it will give a max torque of 26.5NM at the max 25A POS the IONI Pro HC can deliver, and you need a minimum 48V 550W psu.
With an IONI Pro, you are looking at a max of 19.1NM with a psu rated at 510W advised.
Servo would be quite limited with a 24V psu though, so stick to 48V for the best performance from that one. The response rpm will be quite ok with a 48V psu, it is rated 4,000rpm at 640V, but with a 24V psu, you will be slowing down the response to much, it will act as if you have a slew-rate filter applied, I suspect. 48V will put it in the lower threshold of what I would consider a good responsive rpm behaviour.
Anyway, if you had more current available from the controller, it would be good for 38NM with a 48V psu.
Cheers,
Beano