I installed the 3 piece Brace Bundle on my GXP at the same time so no idea as to which item is the most effective. I waited about a year after buying the car before installing the bundle so was familiar with the stock cars handling traits. After the install, all I said was WOW. I could feel a huge improvement in the way the car handled a quick left and right turn such as a slalom at an autocross or a short S turn. No more mushy/unsettled feeling. Other than the Brace Bundle, the rest of my suspension is stock. As others have mentioned, the GXP now handles like a go cart with the braces.
I installed the 1/2" Backbone and it really stiffens the chassis. A good visual experiment is to jack up the front corner (at the proper jacking point with a puck) and measure how much the frame has to be lifted before the back tire comes off the garage floor - before and after installing the Backbone. The back tire comes off the floor a lot sooner with the Backbone. A stiffer chassis does not absorb as much of the uneven road surface since body flex is minimized and allows the suspension to do more of the work.
My 1/2" Backbone is probably overkill, a little too stiff. A 3/8" Backbone would have been adequate. But the 1/2" Backbone looks really cool with the cut outs, but you gotta crawl under the car to see it.
And I don't think that the stock GXP chassis and suspension is that bad. Many car magazines testing the Solstice gave it praise for lack of chassis flex and cowl shake. Drive any affordable first generation convertible and they all experience a lot of flex and cowl shake. It's the nature of convertibles. Without the roof structure, it's really tough to build a torsionally stiff convertible. Old MG's, Triumphs, Healey etc. were very flexible. To race them back then, first thing to install was a rollbar or cage with tubing tying the back of the car to the front to reduce the flex. And rollbars were not required at a lot of tracks back in the 60's, 70's.
I'm sure that if the Kappas had remained in production, the second generation would have addressed the issues.
The first generation Miata was not perfect, but got improved with every generation.
Last edited by syjos; 12-15-2018 at 08:12 PM.