This is for all those folks who can't justify 1,000's of dollars for mods that might get you 10 or 15 hp, but would love to find out about something cheap that provides positive results.
Several companies sell ratcheting bars with pads on the ends for separating and/or securing cargo in pickup beds. They're pretty much horizontal jacks you can load in tension between the walls or wheel wells of a pickup truck. They sell for $40 or $50.
These bars can be used between the shock towers of the BRZ/FRS to provide added stiffness without paying big bucks for bolt-in braces.
I picked one up at Northern Tool and Equipment mostly to give me something to attach loose cargo to and prevent it from sliding around. I was pleasantly surprised to find it actually added a noticeable amount of stiffness to the rear of my car and decreased rattling going over large bumps.
I'm sure many will say this couldn't possibly provide any noticeable benefit, but you'd only be out 40 bucks or so for something that will help secure loose items in the trunk even if you don't feel the improvement I do.