I have a 2009 Cabrio S that I recently purchased. Truth be told, I would have preferred a hard top, but the car had less then 30,000 miles on it and it was priced right... I dont mind the way it handles now, but if I were to say add a larger rear sway bar, strut tower braces or lower the car via coilovers how would that affect chassis flex in a convertible? What areas should I focus on to stiff up the chassis (eg.... front, mid subframce cross brace, tie bars...etc)

 

A rear sway bar for sure would bet your best bang for the buck to improve handling.
https://www.waymotorworks.com/wmw-rear-sway-bar.html

To stiffen the car our X brace is the most beneficial.
https://www.waymotorworks.com/tsw-xb...6-r55-r57.html

A strut tower brace won't be much change, but I found the AEM to be the best, and work with their intake later if you add one.
https://www.waymotorworks.com/aem-st...7-r58-r59.html

 

+1 on RSB
Mine got KONI FSD struts at the same time. An entirely different vehicle that what went in to the shop and one fun kick in the butt ride to challenge the twisties.