I noticed a big reduction in slop after installing the
Whiteline Rear Subframe Insert Bushings
They are especially noticeable at the track when exiting corners with lots of throttle or when launching the car.
[ame="http://www.youtube.com/watch?v=IDfJ_zYFPcs"]BRZ / FT86 / FR-S Whiteline Subframe Lock Out Kit - YouTube[/ame]