Trailing arm Bearings vs. stock bushing
 

I am going to coil overs and for the rear trailing arms I was considering replacing the rubber bushing with bearings.

I also will use a new 935 spring plate.

Is this over kill for a car which is dual purpose track/street?

IMHO, inner trailing arm monoball bearings are a no-brainer with your setup.

Coils and 935 spring plates is a serious setup. To pair that with rubber bushings doesn't make a lot of sense. Sorta like skipping to the advance stuff without covering the basics.