Here's a MUCH easier way to note chassis deflection. This only works for the UNIbrace XB however. Take the XB off, jack up the car on one side and then see how far the bolt holes have shifted. But as Robin noted, this is about feel empirically. Something you definitely notice when driving the car. 
That assumes the unibrace doesn't flex at all when in service. I'm not doubting the device stiffens things up, just that I'm genuinely curious to understand how much.
....As a single panel lattice-cut structure (says he who is not an engineer or architect), which furthermore is anchored and positioned to maximise its effect, it is surely bound to absorb some of the kinetic energy and forces which movement of the car subjects it to while driving. I suggest, but Bruce would know better than myself, that the XB does have a small but insignificant amount of flex just as most materials have. With absolutely no flex whatsoever wouldn't the XB be subject to break or crack? Doesn't energy need to be dissipated?
I would expect a full, or even a half, roll-cage to stiffen up the open box body of a hatchback more than the UNIbraces simply because of a structure which spreads the loads.
Although interesting, for me I don't care what the scientific data is as much as I do about the extra enjoyment to driving which the UNIbraces bring to the party. If you read my posts in the Group Buy thread and elsewhere you will see that I was sceptical about the UNIbraces - I expected the results to be scientific and academic rather than so tactile and tangible.
But as I have already said, having a
permanent stiffy permanently stiffer car and ride is not every driver's cup of tea. Me? - Bring on the bends and roundabouts!
