All of this depends on the car as well. If you have a real Flexible flyer like my old 300ZX, a front bar makes a noticeable difference right away.
It can also depend on placement. From what I have read, the single most effective chassis brace on a Fox-Body Mustang (aside from subframe connectors, of course
) is the K-member brace, providing a tangible difference in feel and control. That's why Ford put one on, along with a strut tower brace, in the factory for the 1994 model, before using other methods in later years.
Even if the brace only makes the car feel more sturdy in the tight turns, that can make you drive the turn faster. It's up to you to decide whether or not that's worth the money.
