I setup a system with 3x G251 running at 3.5A.
Powersupply is the Keling 5010 unit and I measured a 50.6V with all drives active and motors attached. E-stop kills AC power to the Keling and shorts the powersupply thru a 100W lightbulb.
You do get the max performance from the motors/drives but you are on the edge. Having a serious load dump for an E-stop condition is part of your best protection.
I would not power up the G250/G251 without the motor attached and I would want at least 2 drives to share the load. This means you are effectively using the drive & motor as a partial load resistor to cut the open circuit voltage level. Just remember that for this to work the drives must stay in an enabled state. ( I just didn't wire in the disable.)
Another item which can't hurt (added insurance) is to make one each of the back-feed dump circuits using the schematic from the Gecko website and wire them for each drive as described.
A G251 drive with a good stepper running at 50V is a big eye opener if you have only used 'marginal' drives at lower voltages.
Also consider how stable your incoming AC line is, it won't take too much of a surge to put you into the magic smoke region. You must consider risk vrs reward.