The improvements are more noticeable on t-tops and verts, because the cars aren't as tight and overtime will rattle and creak. The SFCs prevent that, tighten up the car's handling and feel.

My car is a hardtop, and I got SFCs put on a few years ago. Handles much better! Thing is nice and stiff, I jack up the front left, and the rear left tire lifts off the ground too! One of the best mods I've done, wouldn't hesitate to do it again if I had another hardtop car. Cheap too, $100 + $20 install.