The Steeda 3 pt Frame rail and TQ box brace was specifically designed with the DC2/S197 in mind. It is what you need. Anything else short of a Watts 5-link will not give you the protection against flex that you want. It also reduces unsprung weight for better launches and acceleration out of turns.
The Steeda parts look sweet, but on their website it says they won't fit on a convertible. But maybe they have other parts in their catalog for 'verts, haven't looked all that closely.
AFAIK, all 2011 Mustang convertibles come with at least a strut-tower brace, though. I would imagine this helps.
I don't have as much car experience at PTM333, but I too have driven several different models of convertibles, from rental Sebrings and Mustangs, to a Saab and a couple different BMWs I've owned. I never felt chassis flex to be a huge issue.
Perhaps professional car guys who are much more attuned to these sorts of things can notice differences more easily. Or maybe if you immediately jump back and forth between a fixed top car and a convertible version, you notice. But on an absolute basis, I haven't found it bothersome.
At any rate, for me, the ability to put the top down and fully enjoy the sun or the stars, more than makes up for any small differences in chassis rigidity!