Regarding the stepped-bore size for e36 m3 MC, I got information from ATE North America (
http://www.showmetheparts.com/ate/). I got my 38mm rear calipers today (91 525i) so as soon as I get them refurbished they're going on and I'll see what difference that makes.
I noticed that Audi A4 cars come equipped with 57 front and 38 rear pistons as well as 23.81 MC... I know that sedans tend to be at least 200 pounds heavier over the rear wheels because of trunks, but wider tires on my Ti will compensate for increased caliper pistons.