Stage

I also forgot to mention tire diameter as that's a big factor in RT. A taller tire staged shallow will give a greater change in RT and ET than a shorter one.

A short tire and a deep stage will reduce RT and ET more than a taller tire will.

RT is driver RT plus vehicle RT added together. Very few people can quicken their RT with a phony little box thingy. You'll most likely find the best RT reductions working with the chassis. You may improve a bit with a RT box, but mostly it's in the chassis.

If you are old enough, you'll remember Scott Geffrion proving this point very clearly to a bunch of doubting idiots.