I have the DNA rear torsion bar. Very well made, very nice product. I bought through TMC but DNA have been good to deal with when I've contacted them directly.

Installation instructions though are non-existent in my experience.

Torsion bar really helps keep the car planted when cornering. It does make the rear feel light, no oversteer on lift off but trail braking you certainly feel it. It helps turn in. Last track day I did was in the wet and the rear got loose a couple of times. It's pretty progressive, didn't snap around, but it an unusual feeling needing opposite lock on a fwd, scared me the first few times!