The FX50 already has a rear sway bar unless I'm mistaken. Just like 1st gen AWD's had rear sway bars. Looking at the pictures, it's pretty thick. I don't know if an aftermarket one would do much. Going to a larger diameter bar isn't always a good thing, it will result in less body roll, but not necessarily better handling. Lots of guys that track their cars run pretty minimal sway bars. The bigger the sway bar, the less independent your suspension is. Get one that's big enough and you pretty much have a beam axle. Large diameter sway bars feel like they make the handling better because the car corners with less body roll. But your suspension may no longer be working as well and you're skating over bumps and losing traction. The OEM FX50 one looks bigger than the aftermarket Hotchkiss I have on my 335. Granted, the FX weighs a lot more, but it still looks sufficient.
The big difference between a brace and a sway bar is that all of the GT Spec braces do is tie together static points to maybe make them stiffer. So it ties together your strut towers, suspension mounting points, etc. By tying them together, it may cause less flex so handling "feels better", turn in is sharper and so on. But I don't know if there would be much improvement in say lap times at the track. The sway bars actually attach to dynamic suspension points so they move. It actually transmits forces from one side to the other. So the bigger it is, the more it transfers and the less one side has to do which is why the car will not roll as much, the suspension is actually compressing less.