I have all sorts of shoes & pedals including MP66 shoes SIDIs Lakes / xt 647 caged 545 caged etc
Sure the suggested combo 'works' fine , but as you realise they are far from ideal xc race shoes. Going from these to light/stiff SIDI Xc shoes I do notice a difference in riding efficiency.
MP66s play best with a caged pedal which supports the flexy sole better, but at the end of the day if you are comfy & satisfied that's all that matters.
I actually do half my riding in Shimano SPD sandals...