Hi Mim, I also have a 2015 car and noticed a very big improvement with the flexible strut tower brace. Maybe after the suspension revision and the updated bolts, the car needed more the tower brace. I believe that you need now better tires and make the rear a bit stiffer. When I installed the strut bar, I noticed sometimes the rear going a bit sidewise. I managed to fix this after installing a bigger bolt and an additional stopper in the rear subframe.
Here are the links:
https://www.japanparts.com/parts/detail/45368
https://www.japanparts.com/parts/detail/45367
Now I feel that my car needs a bit of dialing again in the front. Welcome to the tuning world
. I believe I'll manage this with the longer steering gear bolts. I haven't installed them yet, but I heard that they make also a bit stiffer the front frame. Not too much, but as I said it doesn't need a lot in front.
Here is the link:
https://www.japanparts.com/parts/detail/47821
Regarding the flexible draw stiffener, I don't have an experience in the BRZ. However, I installed once a similar component to my Forester and I couldn't tell any difference. I am sure that you will feel an improvement in the BRZ, but not as massive as other parts. For the time being I want to spend my money to other improvements like exhaust etc.