I think you're talking about deflection and/or shimmy, and not grip or roll like people have mentioned. This is almost completely due to your sidewall. Think about how much deflection the tire can have, in mm (probably 20-30mm). Then think about how much any bushing could possibly deflect even if it was missing all the rubber. The only one that would deflect more than a few mm is the subframe/chassis bushings. Try adding 5psi to your rear tires and going for a test drive. If it makes a difference, you're on to something.