Im not sure 100% after googling it. I assumed it was across the board, particularly as its in the parts for all classes, had a look on reddit and guy said it helped slow the wear in track races... In my experience I found rallying easier after adding the iron grip legend points. Its easy to forget how duff the intial set up of the game is, cars with low part values and default control set up were a bit meh, it soon becomes a real comfy pair of slippers with some tweeks in settings and upgrades though.
Still getting my kicks from snapping away, some really choice liveries in this game.
Iron grip seems to improve cornering, I took the legend points off for a test and it definitely drove worse without it.
The linearity setting should be more important value to adjust if it feels numb, I remenber the game had too much understeer in default state. Persevere with adjusting one value at a time, you will get it dialled in to where you like it ;-)