Steering feel at higher speeds

[Nmbrsix]

Since purchase, my '02 Box has had an odd sense of on-center slop at speeds above 75mph. I've had a suspension refresh including new front track arms, at least one rear control arm (bad ball joint) and track arm, new Bilsteins/strut bearings, new tires (Conti ExtremeSport Contact), and 4 wheel alignment all done by the best shop in the region. I've also had one of the front wheels straightened that was causing some vibration above about 70mph, but the steering feel issue has remained throughout. At any lower speeds the steering feels nicely weighted without this on-center slop.

Any thoughts on this? Is the rack bad? Control arm bushings? I've been trying to ensure the car is ready to do DE days, but I don't think I'd feel confident in it as it is currently.