Seat Back Bracing

Not sure if this is the right forum for this question, feel free to move it if needed.

I have a set of NRG seats (https://getnrg.com/collections/seats/products/frp-311) not known for being top of the line, but they fit my budget and the dimensional constraints I had. On the track, I've noticed how much the back of the seat flexes around corners, The bottom holds my hips just fine, but the seat is flexing side to side a good deal and I end up using the steering wheel for bracing my body more than I want to or should.

There is a harness bar behind the seat. Can I drill a couple of holes in the back of the seat and brace it to the harness bar, or is that a bad idea? Is this gonna compromise the seat in the event of a crash?

I just got my first RaceTech head restraint seat delivered last week (hasn't been mounted in car yet), but I was shocked at how much/how easily it "twists". It does have back mount provisions and I intend to take advantage of them, but I guess my past seat experience has all been with seats that were metal frames, not composite...I'm assuming that for $1k+, if it flexes, it's supposed to flex, right?