The Boss is old news, not competitive with the ZL1 or Z/28 and maybe not even the 1LE. It is also no longer produced.
The competition for the Z/28 hasn’t been announced yet. I suspect GM is leading the way in anticipation of a better performing, track oriented Mustang. Or maybe the Z/28 is so unique it will go unanswered through the release of the 6th gen Camaro in 2015 (MY 2016).