Having never owned a Mustang I can't voice an opnion on em but.......
I have had two Corbins and would agree that they are Hard and take awhile to "Break in" but so does a new leather recliner. The reason the Corbin is hard is so that the pressure points don't develop when your sitting for hours on end.
I would submit, that on "soft" seats, after about 3 hours you start feeling spots on your behind that Hurt, so you start moving around in the saddle trying to get off em and then where ya moved starts hurting, pretty soon, it don't matter where ya move.......it hurts, not very long after that you gotta get off the bike, period.
While the same is true for "hard" saddles, the difference is that they seem to spread out the weight and take a lot longer to develop the pressure points and once you've given your seat a little rest you can get back on for another hour or 2 without going into the "hurt" stage.
Mike Corbin put it this way......."There's a reason them old Cowboys and saddle tramps didn't have pillows under their a(self edit)s back 100 years ago!"
And lastly, please check the Iron B_tt Assosiation registry, mainly the 11,000 mile marithon. Several years ago, around 02, the 10 or so riders, that participated, were given a new saddle at the first 5 stops, they had to use it for 1,000 miles and at the 6th stop they got to choose any of the saddels they had use to finish the marithon, 9 out of the 10 chose the Corbin, these guys are riding 1,000 miles a day for eleven days on every kinda bike ya can think of. If they liked the Corbin over all of the others, and, I honestly don't remember weither Mustang was in there or not, that's sayn a lot. Corbin for me!