It's a combination of power, accuracy and............presence. I've never tried MP-300 but I have maxed out some W296 loads. Lots of noise and fire. Same with H4227 and 2400 when loaded heavy. I settled on 2400 at just below max for magnum rounds. I just like the way they shoot. Here's non-partisan table to look over and, as always, be careful.
http://www.reloadammo.com/357loads.htm