Hey guys! I might as well jump in here...
I bought my '98 GT vert exactly 10 years ago tomorrow (3-12-99). The very first real mod I did (besides taking off my silencer) was I bought the Kenny Brown Super Subframe kit. It had super subframe connectors, matrix braces and jacking rails... (The bend in the subframe is intentional)
Chris, my jacking rails make it so that when I jack on the side toward either end of the car, the entire car lifts without bending...!
(Some of you will remember Kenny Brown as being an exceptional company until there was some internal theft that ultimately shut the company down)
The first thing I noticed was that the doors and windows on my convertible quit going out of alignment under hard driving.
The second thing I noticed is that the "Understeer" that is common with our stock Mustangs becomes more of an "Oversteer". I prefer that. I can come blazing into an off ramp loop and take the corner very well at high speeds. Also it's great for drifting if you are into that sort of thing.
Every convertible should have subframe connectors. And putting them on coups isn't a bad idea either.
The picture below is what my subs look like mounted, although those are not my subs or my car in that picture...