I actually have something to contribute to this thread! I installed the cowl extension and cowl-to-strut tower brace and the lower k-brace that's part of the PP1 package, the 20" wheel option and the convertibles, but not part of the non-PP GTs. I'm hoping this will tighten up the body enough to reduce the "wallow" that I feel in corners. I'll have to wait to test it, as I finished the install just as two days and nights of heavy rain started falling. I also got a set of 265/40R19 Michelin Pilot Sport AS3+ mounted on OZ 19X9 Leggera HLTs that I put on a few weeks ago. The OEM 18s that came with the car will soon have dedicated winter rubber that I will use during the snow and ice season.