FWIW - Schoeps does it Mk4 + Mk8. They must know something. I am pretty sure they arrived at this combo as the result of extensive testing.
From the Schoeps literature - mk21: "The low directivity of this microphone type prevents it from generating large enough level differences for X/Y use,
though M/S and near-coincident spacings are often successful." "When used as a spot microphone it must be placed somewhat closer to the sound source than a cardioid would be, due to its lower directivity"
- mk4: " When used for X/Y recording or as a Mid microphone in a Mid-Side configuration, it delivers exceptional results" "Due to its directivity, the MK 4/ CCM 4 can be placed 1.7 times farther from the sound source than a pressure transducer of equivalent sensitivity, while maintaining the same proportion of direct and reverberant sound"
so, schoeps does recommend the 4's for an M/S recording, and would probably work better further away from the sound source, where the 21 would probably work great in FOB placement where you are closer to the PA.
My personal preference from the tapes I have heard favors the 21's for sure...... I love the 21's sound and really love that extra kick in the pants on the low end. Maybe try to find tapes made with both and see which your ears favor. I would bet you will have sweet results with whichever combo you choose though.
Edit: A quick search on archive returned many results.
http://www.archive.org/search.php?query=schoeps%20M%2FS