The UMI and Hotchkis brace is a beefier version of the old factory braces that used to be on the A-body chassis. They looked very similar to the rad support braces in our G Bodies.
From a physics point of view, the bolt holding each control arm goes through the frame and holds the control arm. The Threaded end of the bolt is not connected to anything so the bolt is essentially a cantilever with a very weak sheet metal brace (the control arm mount). When the control arm moves around so does the bolt. The brace adds a rigid member to the cantilever end of both bolts, and make the bolt a bit more rigid. So when you launch, the mounting points of the control arm are more likely to stay in place rather than flex. The upper control arms are mounted slightly better, but ultimately that cross member is just a piece of sheet metal. From the looks of it, I think the brace MAINLY helps the lower control arm, and not so much the upper.
In terms of weight, the benefits of the rigidity far outweigh the detriment of the insignificant weight gain. You could also make it yourself out of crushed tubing if you were so inclined.
As the others have said, anything you can do to make the chassis more rigid is beneficial, especially with a sub 1.7 second 60ft time that you will definitely get when running in the 11's.
Just to give you an idea. My previous car was a 1971 Buick Skylark with the Buick 350. All big block buicks got this chassis brace from the factory. But my 350 car produced enough torque (with a bit of carb and timing adjustment) to flex the frame. My car sat lower on one side than the other after i was done tuning and thrashing it a few times. It ran 15's in the 1/4, but it snapped your neck on launch from torque. This is likely to kind of damage you will see . The lower control arm mount is definitely the weaker of the two. But the upper mount is also a weak link.
Hope that helps. (Attached is a picture of the A-body Factory Version just to give you an idea).
