A TON? Like better springs and shocks? The roll cage was for driver safety not to stiffen the chassis. That was done to the Chargers too. Magazine articles about the movie/car(s) didn't come out until years later. Peoples recollection of what was done probably varied greatly. Yes, the car was trashed afterwards. Every body panel was dented and I am sure the subframe was twisted. It cost more to repair than the car was worth (then).
HowStuffWorks "Mustang Stars in Bullitt"
"Two identical Mustangs and two matching Chargers were used in the Bullitt chase sequence. So that the four-speed Mustang could run more easily with the brawnier four-speed 440 Magnum Charger, Hollywood engineer Max Balchowsky installed a racing cam on both Fords, milled the heads, and modified the ignition and carburetion systems. Additionally, Balchowsky bulked up the suspensions of all four cars for improved strength, handling, and control. One Mustang and one Charger were fitted with a full roll cage."
Go drive a 2008 Mustang with a V6 and a V8. Are you going to walk away from the test drives going "Wow the V6 Mustang with the 100lb lighter engine handles so much better" or "The HP and Tq from the V8 was awesome."