The roll cage is more of a preemptive safety item ( some tracks require a roll cage ).
The GT350R .. all of the SFC and RC were installed at the same time; but the car was tracked before and after - there is a noticeable difference in body flex, but for the most part the biggest limitation on that car is the tires - the Pro Cup2s wore out very quickly and it has regular summer tires for the time being. I don't believe the one brace in question does a whole lot except in hard driving ( remember .. the Shelby has upgraded bushings over the GTs so that point of flex has been eliminated from the factory .. also, the GT350 is a "wide body" so it's not exactly the same Mustang chassis to begin with ).
The 911 GT3R got the RC immediately after purchase - the owner of that car also has a GT350R decked out with the RC and all SFCs but it's been sidelined in favor of the 911 for the track.
My SS has a noticeable creak from the rear and front windows driving over surfaces that flex the chassis - I have been told the SFC will cure that and I intend to find out with my next round of upgrades ( SFC, Tunnel Brace and 6 piston front Brembo calipers are next on my list .. )
As for tying the subframes to the chassis on the Tbird; absolutely makes a difference. I did an installation for a member here years ago, but to sum it up .. vogtland springs, KYb shocks I think, front SFC and the crown jewels, the biggest ADDCO front and rear sway bars ) .. that car drove like a go kart on rails after those mods, when you turned the wheel, the whole car turned with no chassis roll - I believe the biggest contributor was the sway bars on that setup.