Folks, a modern industrial process for tolerance matching would never use hundreds of bins, that is so last millennium.
In modern manufacturing, this type of problem is typically solved by measuring part A size (piston or piston ring in our case) then auto-barcoding the tolerance on it, then storing it in a known location in an automated shelving/delivery system.
Then come assembly time, part B (the cylinder bores in our case) can again be measured in real time, then a specific piston or ring can be selected for each bore from the pool of available parts.
This no binning system would have the capability to match pretty much any tolerance given a large supply of pistons or rings, which BTW is exactly the process of matching parts that an engine builder does.
It's a simple workflow which can be implemented with a few sensors and an automated shelving/warehousing system. Now I'm not saying this is what BMW is doing because I am not privvy to their secrets, but if it would provide such great benefits then they could easily implement something like this to offer tolerance matching that can be easily on par with, if not better, than the corresponding human process.
My impression is that any potential benefits are overstated. I mean, are we talking about the last 10%, 1% or less? Nobody can tell...
2015 F80 ED
2004 Z4 3.0
2014 X1 28i ED