I had the KB super subs, matrix brace and jacking rails all installed together, so I don't know how the car would have felt with only the subs, but I'm extremely happy with the way the combination turned out. I'd bet my pink slip that my ragtop's chassis is significantly more rigid than a stock coupe's. I no longer get any flex or twist and not a bit of cowl shake, even at 150 MPH.
You should be aware that installation isn't just a matter of welding up the pieces. Because of Ford assembly line tolerances, KB sends the parts out slightly large. It took the chassis tech at my regular body shop a full six hours to complete the installation, because he had to cut everything to fit properly on my car and needed to notch the matrix braces in a number of places to avoid pinching fuel lines, etc.
Edit: I've just found the Microsoft Word document with the photos Mike Dempster shot of his KB extreme matrix installation, showing all the notches and scallops he added to the parts. This is the same doc my chassis tech worked from during the installation on my car. If you want a copy, I can email it to you.
You will love the jacking rails just for the convenience of them.
I absolutely love what my Kenny Brown pieces have done for both the look and handling of my Cobra. I have the following from KB: extreme matrix brace, super subs, jacking rails, strut tower brace, rear shock tower brace, club sport springs, cc plates, irs foward torque brace, and the polyeruthane differential mount bushings.
I picked up my '02 GT yesterday from Kenny Brown's. I had them install all you mentioned plus: sub-frame connnector, strut tower braces and shorter springs.