Simple answer: the trunk brace is not a strut tower bar.

The rear shocks and springs are neither supporting as much weight as the fronts, nor are they as stiff as the fronts. As a result, a lot less force is exerted upon the rear shock towers.

For whatever reason lately people want to buy up every stupid reinforcement bar they can get their hands on as if it's going to make a huge improvement in the way the car drives. Basically the stock GSR front strut bar is good enough, if the car doesn't have a stocker then something like a Neuspeed front bar is nice. With an upgraded rear sway bar, a rear lower tie bar is probably a good idea.

There are however, far more important things to concentrate your hard earned dollars on that will make much more of an improvement in the way the car drives. They are: tires, shocks, springs and sway bar(s). A good alignment never hurt either. Until all of these things are taken care of I really wouldn't worry about getting your hands on the lates "gee that sure is cool" structural reinforcement.

The worst part of all of this is a simple roll cage would be way cheaper and way more effective than all these weird bars bolted to random places. It's like now you have to have a front and rear strut tower bars, front and rear lower tie bars, a trunk bar, a c-pillar bar, blah blah blah. If you really want to stiffen up the car and don't mind a bunch of bars all over the place, save the hassle and get a cage.