I don't think that the "Mustang guys" are clamoring for a track capable car any more than they want a drag strip brawler. I haven't seen any huge influx of S550 Mustangs at the track over the last few years. In fact, I don't think I've seen one. And I've seen just as many GT500's as I have Boss 302's. I think there is more talk about the track than there is actual track days. The vast majority of the GT350's will never be on-track, same as with the Boss 302's, Z28's, etc..
Ford didn't have a problem putting a bespoke supercharged 5.8 liter Trinity in the GT500, and there's no reason they won't do the same for an S550 variant considering how well those cars sold.
S.
Well, the S550 has only been on the market for barely over a year. I still rarely see one driving around. You need to wait for market saturation before you can say whether or not S550 guys are (or will be) hitting the track. Also, don't forget that road courses aren't exactly a dime a dozen. I don't think there's a single one even within 2 hours of me. That being the case, the vast majority of track capable cars in the U.S. never see a track, but they spend plenty of time on back roads.