I don't think it's true that the thin walls of the M-Geschoss shells limited velocity that much. Not only were the MG-FFM's shells fired by the more powerful MG 151/20, but (more surprisingly) the low-velocity MK 108's shells were also fired by the high-velocity MK 103. The base of the M-Geschoss shell was hemispherical, which is the best shape for resisting the pressure of firing, so made it stronger than the thin walls would suggest.
Tony Williams: Military gun and ammunition website and discussion forum