every gto has rear squat, does it mean you need springs/shocks, no. does it make for a rocky ride yes, does it handle like shit after 30000 Mi yes. are they needed, no. over 30000 miles every gto needs new RR bushings, and if your gto has actualy not had a RR bush problem. Definatly worth the change either way because the difference in steering and braking delivers a new confidence in the drive. Those parts suck, and they only get worse, can you deal with tthem yes. but its not that expensive of an upgrade and makes all the difference in the world in the drive.
Theres many of other parts that most say "need to be changed" but reallly, nothing NEEDS to be changed unless its broken. But i couldnt deal with the effects of those crappy parts so i felt the need to change them. after 25000 miles my car hated all turns, the body roll was ridiculous on long or sharp corners and the steering was so sloppy i wanted to blow it up. now i love it again

... of course im not done with the suspension, but those mods make the car alot nicer to drive in all aspects and i spent maybe 700 installed. If you dont care about "The best" handling then it makes the gto perfect as it still handles good, dont feel like its going to tip over at parking lot speed and the steering feel really awakes with the new caster settings
So, the real question is. what drive feeling annoys you most about your gto? for me it was the rear squat, steering response, and body roll. cause as far as im concernd, even though all those parts were still working, they were still broken.