I just bought my 3.91 ring and pinon as well, and am getting them treated. (Thanks to Beta, who has been a great help with sourcing the gears, and with the treatment and associated legwork and research.) These are "before" shot peen and REM polishing.
I'm shooting for an install early next year (still need to ship for polishing and just got the ring and pinon).
Good description of the shot peen process
here. The REM polishing/treatment creates a micro layer of treated material reduces friction (reduces wear and frictional losses). IIRC, the 4.1 gears from BMW are shot peened, while the 3.91 are not.
Beta will have the give the exact cost for the peen and REM. Should be somewhere about $125 based on my discussion with the company (range is lower and higher based on size of parts). Company is
Weddle, near Santa Barbara.