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@Nick_S
The grade of bearing refers to the shape of the ball, rather than its hardness (which is measured by the HRC value) , with larger grade numbers indicating greater departure from a perfect sphere:
https://en.wikipedia.org/wiki/Ball_(bearing)
The ball bearing entry in the late Sheldon Brown's cycle maintenance Web archive at Harris Cyclery suggests that Shimano Dura Ace and Campagnolo Record use grade 25 ball bearings:
https://www.sheldonbrown.com/harris/bearings.html
In reply to the question raised by @ElephantBreath on harder vs softer materials --- Park Tools' Web site discusses the issue of ball bearings in the context of ceramic ball bearings being harder (HRC 75) than the bearing surfaces of the cup and cone system (HRC 55-60). They suggest that higher hardness values for the ball bearings "should provide good wear features for the entire bearing system, not simply the ball bearing itself. ":
https://www.parktool.com/blog/repair-help/ceramic-bearings
So from what I can see, the grade 10 chrome steel bearings with hardness of 60-67 HRC offered by Simply Bearings should well exceed the requirements of all qualities of cup and cone bicycle bearings, for what seems a reasonable cost.