If you put poly in the rear, you just make the binding even worse. Nothing back there rotates on a single axis, everything has 3 dimensional movement. As the trailing arm goes up and down, it also moves side to side. It all moves in an arc. I mean you can do that if you want and it will be stiffer, just for the sake of being stiffer I guess.
I have Powerflex bushings in the rear, they seem like well made bushings. The rear picks up quite a bit of traction with them installed, but as GSM says, they resist movement in any direction except up and down so they can make the rear feel like an older twist beam with a big anti-sway bar.
You gotta be careful with rear modifications, my car was at a point where an inadvertent small jerk of the wheel when at the edge could cause the rear to cut loose abruptly, even with softish springs. This was at a much higher cornering limit than stock, as I also had the front spindles. Backing off the rear bar might have helped some. Its possible resistance to movement in other directions caused issues, but I suspect rear bearings will be even less forgiving, although they should feel better since they'll articulate in all directions freely, and ultimate grip likely even higher. I'd suggest a little toe in on the rear to start with bearings, then when used to them go to neutral, then perhaps to a little toe out.
034 makes rear bearings, might be worth checking with them to see if they have any plans for the fronts.
