I don't get it. CNC is the surface finish right? It doesn't tell you how "reliable" a cog is. Also, being stamped won't probably make a cog softer or anything. Hardening is probably done after stamping, because it doesn't make sense to stamp a hardened steel. Thinning is probably a problem, but you know people use 3/32 cogs right? I think those are wicked thin, but they don't break do they?

That being said, most CNCed cogs are identical and you don't need to inspect the cogs to guarantee that you are getting just right. Some stamped cogs so uneven, you lay em together and you can see they are significantly different. I could pick up a CNCed cog on a fly and expect reasonably identical performance.

Drive efficiency we need to ask people like Carleton who do real track work. I am pretty sure even flat contact surface directly results in performance improvement. But then for those people, I am sure, the flex of chainrings also matters. ... Totally different world to a casual street cyclist like me.

P.S. Lately, I am beginning to see really cheap CNCed cogs. Like the one from Steelwool. Those sell like 15-7 in Tokyo. Beats my stock stamped cog by 5 bucks. Geez. Those any good?