I'm surprised you haven't already gotten the most common/best answer, the way it sounds is up to you, I personally don't like x-pipe at all. A mid-pipe is a mid-pipe, the only things that will change the loudness is the diameter and cats. Assuming that both are 2.5" diameter, you may be able to tell a slight increase in noise, but not a whole lot from just losing the cats. Unless that box on the MAC actually does something really different than a normal x-pipe.
I would say save your money and get the cats cut and a piece of pipe welded in.