What you get with the clipped 13X6 prop (to make it 12 inch dia) is larger blades than the unclipped 12X6. More blade area = more load on the engine... until you hit max static thrust (or max thrust for the airspeed)

Assuming you have shaped the tips appropriately... you get the same top static thrust, but at a slightly lower rpm. (because static thrust will max out based on dia, more than blade shape or pitch.)

It is rue that just chopping it off square ended is a bad idea.