I found the 4-piece Summit version to be an absolute revelation on my car. I ran the seperate pieces at different times to see the full extent of the differences, and I found the following:
Main cross brace: Took out a lot of the flex and vibrations of the car (but bear in mind this is on a Roadster), car felt tighter but not hugely different in corners.
+ front lower brace: Sharpened up the steering by A LOT, and I cannot emphasise that enough. The car became pin-point sharp in the corners.
+ both rear braces: With the rears on as well, it gave the car almost a rear-engined 911 feel as you could feel exactly what the rear end was doing, which gave far more confidence on getting the power down coming out of corners.
For a coupe I'm not sure I'd bother with the cross brace at first, but certainly the front and rear parts should be a must-have on anyone's list of mods to do. I would get these bits done before any major suspension changes (damper or spring) and see what you want to do from there. Tbh it's a bit of a surprise that these aren't considered a must-do mod by more people on here, but then seeing as how most people buy the car for the looks than spirited driving I guess that's not too surprising. In addition to what Greekman says about catching the cross brace when lowered, I can confirm that you'll get this on cars with standard suspension and stupidly high speedbumps too, however it's quite minor and if you drive slowly all you'll do is scratch it and give yourself a fear over the noise it makes!