Thanks for all the informative replies!

In my understanding, 'high-grade' cotters are made from a harder steel and have flats and threads that are machined or ground as opposed to the lower grade type with stamped features. Those are made out of a softer steel to allow stamp-forming the flat, and roll-forming the threads, and are made from a slightly undersized rod (9.2mm vs. true 9.5mm) so that the stamping operation doesn't cause them to expand beyond 9.5mm which would cause fit issues.

From a mechanical design standpoint, a tight-fitting cotter leaves less of a gap for material spread when the cotter is deformed either from insertion or the dynamic loads generated while pedaling. Also, a cotter closely matched to its bore gives a better distribution of stresses.

jonwvara hooked me up with a set and I have a good mill file to tune the flats, then I'll be using one of these to press them in $16 Cotter Press

I'll report back with the outcome!