As I recall, the # grade of the bearing is not only the precision of size and an indicator of material quality,
but the the consistency of size relative to the other bearings as well. All the balls are within a given tolerance
of size to the others.
The lower the grade number, the higher/closer the tolerances.
That is why there are more expensive, and IMO, worthwhile for precision replacements.
All things being equal, a grade 25 bearing (Campagnolo quality) in a precision setting will likely be smoother, and
have a longer life than a grade 300 matched set up.
Replacing a grade 25 bearing with a grade 300 version will likely cause pitting of the cone and bearing to be accelerated.
Replacing a 300 grade bearing with a grade 25 bearing in a non precision setting( soft, non ground cone and cup) will not offer
any appreciable benefit, although it will work well. It will not stave off the wear inherent in a non precision set up.
Wear is a function of the least precise part and ground surfaces are much more precise.
Low end cones(softer, non ground, chromed) and low end bearings(grade 300 ) will be common on bicycles, but are more
likely to pit the cones and break down the bearings- even if there were properly adjusted.
It is not that "Campagnolo quality " cones and bearings cannot pit, but would be generally less likely to if properly
assembled, adjusted and with occasional service.
Impurities in the grease from use(dust, dirt) or metal particles from the manufacturing process that were not removed
before assembly are the bane of longevity.
Check the grease out of a low end hub, headset or crank, especially if they use retainer type bearings, after years of use.
You will likely find there is some metal particles from the break down of the bearing and or cone.
The grease will have a metallic color and will feel a bit "rough" when you roll it around on your fingers.
The surfaces of the bearing will also bear a different finish than a precision ground, better steel grade set up.
rusty