Let me start with the notion that terms like "stretched", "squeezed", "pinched", etc., should be defined with respect to the tire size's measuring width wheel rather than visual appearance (which tends to be subjective). On that note, I'll go along with the 255's being stretched (only slightly so; I wouldn't be the least bit bashful about putting 255/40's on max-recommended 10" wide wheels)
For your 275's, "neutral" is a pretty good description as long as you realize that that implies a more balanced combination of driving/riding factors then what you're chasing as soon as "better handling" is what you really want. The 285/35's should have been inherently stiffer laterally than the 275/40's, but on 9.5" wide rims they were half an inch below measuring width, which gives away some of that stiffness.
It's possible that in rapid maneuvers, you're hitting the bumpstops hard enough to spike the load on the outside tire, which would make it run at a higher slip angle. That's separate from the older tires perhaps being heat-cycled out.
Your tire choice probably needs to consider whether or not this car needs to be driven in cold weather (or snow), and whether or not you're willing to run one set for "3 seasons" and a second set for the winter.
In a corner, the car will roll (duh), while the axle more or less doesn't (actually, it does "roll" a little due to load transfer compressing the outside tire a bit and letting the inside tire un-compress from its static weight position). This difference in angle needs to be accommodated in all three rear control arms, preferably without inducing large loads and greater load transfer. The OE solution is soft rubber bushings with shapes (and holes /voids) that permit this motion without developing much force. Anything stiffer needs to allow all of the sliding and rotations between individual components inside the joint.
Right now, my car has BMR's handling spring kit that was specifically for GT500 applications (about 20 ln/in stiffer than their current GT/GT500 handling spring kit). And I did shim the rear up by about 3/8". For rear LCAs, I've got a set of Currie 'Currectrac' LCAs that feature a Johnny-joint at one end and polyurethane at the other. I DIY-modified the poly to make the arm work even more freely.
www.scandc.com if you're interested and if Marc can still get them (he's mostly a GM intermediate guy).
I'm going to try to avoid typing things twice.
Norm