[casioqv]

It's hard to argue with real world experience, but that is really surprising and confusing. I wonder what is really happening here?

Perhaps rather than preventing the cornering deflection that normally occurs in a stock car without the brace, having the brace causes it to always be "fully deflected," even in a straight line, so you don't get the abrupt transition/shift that makes the car feel less stable.

I'd also expect the factory alignment to anticipate the shift caused by the cornering load... so if compressive loading stabilizes the suspension better, perhaps you'd only get the benefit by also altering the suspension alignment such that it's no longer pre-compensated as heavily for this shift/distortion?