[Moto-P]

Might be raising a dead thread, but I thought I would add what I am finding in recent years. Including my own car, a 2013, several cars in my client lists exhibited the same things in recent years.

The upper arm of the rear suspension was showing significant wear on the bushings and joints which will allow the wheels to wiggle under loads. It may not be apparent in inspections on a lift or jack stands as the upper link is canted enough to not allow it to wiggle much at all when the wheels are drooping. But removal shows clearly loose joint at the hub.
This is on a 70,000 mile old car typically and is something common to cars that have decent grip tires, and in the roads of the USA where pavement is rough. It's also typical in most cars, but FRS/BRZ is especially sensitive to such wear as the car is so responsive to driver feel.

Also can be said for, rear control arm bushings, front lower arms and its king pin joint, steering outer arm ball joint, as well as steering rack mount bushings too.
Though the effects might be less apparent in freeways, a replacement of these parts, will refresh the car to that nice taught laser sharp feel again.

What's happening is that even with pin-sharp alignment done at static state on a rack, as soon as it sees deceleration or acceleration on the highway at speeds, the wheels either toe out or in, and steers the car left and right under different throttle loads. This because even with the steering and toe arms holding the wheels tight, the upper joint at the rear is still allowing tires to change alignment at the root mount. Same can happen with front lower joint but it seems to be much more annoying and apparent when rear wheels steer out of alignment as it does so without any steering feedback.

Hope this helps.