Ran a 14x4 on mine on a 6lb 110z cap plenty of vertical power and good throttle response. Was going to try a 15x4 but sold the plane.

A 14x6 prop has a prop factor of 34
A 15x4 prop has a prop factor of 34
12x8 prop factor is 32
props with the same prop factor take roughly the same amount of horsepower to turn the same rpm, ive been using this formula for a while now and its not uncommon for an engine to pull a slightly larger prop factor than it was designed for and pull it well.

prop factor = diameter X 2 + pitch

Your right about the mds design its made to turn bigger props with less rpm's this is partly done with a longer stroke on the crank.