I have mentioned it before but here is a list of items that only the Mach 1 could have for options.
# 1: Special Color_Keyed Carpeting, plastic runner in the carpet.
# 2: The word Mach 1 in the tape stripe and on trunk. You could get stripe on any model with no Mach 1 verbiage.
# 3: The Mach 1 Sports Interior Option. Little fancier design.
That is all I know of that you could get on a Mach 1 that you could not get on any other model.
Are there any others I missed? A Mach 1 was not any more special than any other model unless you paid for the extra options that you could add to other models as well.
IRT your #3: I don't recall the Mach 1 having anything specific in the interior that couldn't be had with other models, aside from a "mandatory" clock and the vinyl carpet inserts. The Deluxe Interior could be ordered for any model, as well as Mach 1s could also have the Standard Interior package. I'm not familiar with a Mach 1-exclusive interior package.
As far as 'sleeper' look goes, I don't think there's anything 'sleeper' about these cars at all. It's not a Farm Truck or Grandma's Country Squire Wagon with a dual-quad 460, after all.
When I see a vintage Mustang Sportsroof, I more or less "expect" to see certain things - most of them involving muscle-car equipment (cool wheels, fat tires, wings and spoilers, engine goodies, etc.). But, I've also learned that you never turn your back on anything turned out from that era just because it's factory stock in appearance and wearing hubcaps.