A-904 shift kit

FWIW: Kolene isn't a coating, it's a metal treatment. The dark/black finish is a post treatment surface finish (black oxide), but the process is similar to case hardening (but with less warping). It's akin to a salt bath nitride operation and increases the surface hardness of low carbon steel to a significant degree by diffusing nitrogens into the surface. This can do several things: increase wear resistance, increase durability, reduce friction. Depending on how the process is applied, the reduced friction could obviously be a problem in a transmission but I'm assuming those that offer them know this and so they don't over-polish the parts (part of the kolene process is to polish the components - this is done to reduce the chances of intergranular corrosion, and rust in general, developing). If over done, the polishing can reduce the coefficient of friction pretty substantially, but it's also common to skip the polish.

The difference between a kolene treated and non kolene treated part is that the treated one might last longer if you run with low pressure. The clutches still die first, though. The other advantage would be the increased strength which would make it harder to damage the edges of the drive tangs on the steels. I doubt this is much of a concern for street cars though.