I use/used Channel B for individual module binning as well. But when I tried 1600c11 in B+D a.k.a. "the hardest slots", I couldn't pass 32M no matter what. But it worked well in A+C.
On MSI, A and C have looser RTLs (47/8 at 1600c11) compared to B and D (45/8 at 1600c11), this is what has most likely made things easier for mems and/or IMC.