The OP was asking about availability of the Kicker 10" trunk mounted subwoofer, and yes its been available for almost 2 years now. The article in 5.0 Mustangs & Super Fords looks like it was recycled from a while back just to remind Mustang owners of this option.
To the OP, check out PointNarf's write-up of his install of the Kicker 10" sub, it's a pretty decent overview: https://www.allfordmustangs.com/forum...b-install.html
Also, like others have mentioned, if you go with the full Shelby Kicker upgrade and replace the 8" door subs you will notice a difference in "boomy" bass inside the car. The factory 8" door subs in the Shaker 500 and 1000 were quite muddy and boomy and quite frankly sounded like crap! The 8" door subs in the Shelby kit definitely tighten the bass but are noticeably quieter (probably due to the 2 to 4ohm change) but all in all are 10 times better quality than the factory paper crap 8 inchers that come stock.
If you decide to go with the Shelby Kicker upgrade kit then I would highly recommend that you pass on the front 6x8's and go with a quality name brand you prefer. The 10" Kicker sub in the trunk is surprisingly adequate for moderate levels of bass -although that's an entirely subjective statement. I bet you'd be pretty happy with it.
I bought mine just to fill the car with a little more bass and was more than happy with the Kicker 10". I also wanted to maintain as much stock radio functionality as possible and this kit totally delivered! Yes, for the same money I could've purchased better components but then I'd have to give up the stock features I wanted to keep.
After nearly 8 months, I'm just as happy now as I was the day I installed the 10" Kicker trunk sub.