[Cyberdemon]

Aftermarket suspension can be great, but lowering springs are generally hacked together pieces that are not properly engineered for a complex, adaptively dampened suspension like the one on your car.

They may be a great upgrade from the stock mush springs on the average sport-compact sedan, but that really isn't the issue here.

Generally you get movement in the rear axle due to the suspension linkages, not the rate of the springs. This is why the M2/M3 have the rear subframe hard mounted to the chassis. That reduces a great deal of the slop which can occur in the rear axle (while increasing the noise/vibration/harshness of the ride).

Usually to improve that condition, things like stiffer suspension bushings and other suspension components to reduce that flex & slop are where you want to spend the money. I am not familiar with anyone doing urethane bushings for the F2x yet though.