Stiffening the front bar and/or softening the rear bar will both serve to tighten the car up (make it less prone to oversteer). They don't do the exact same change since the two bars are sized differently. I.E. one hole change at the rear is not the same as one hole change in the front, so you'd want to try both (maybe even at the same time).

No info on what you are trying to accomplish though, which makes it tough.

As for the softest hole on the rear bar. This is the second system I've found this with. I can't test them all, and it has to do with how the pipes are bent and enter the mufflers. Stock is no issue, no problem with the Magnaflow I had on my 2011 either. No issue with a StainlessWorks system on a friend's Boss 302 either.

 

I wanna go fast...just like Ricky Bobby! Raced this weekend and have a few cars to compare to. The car seemed to like struts full firm front and 1/2 out from full firm rear. Air pressures for the Goodyear f1's were 36 front and 31 rear. Front bar in the middle hole and rear in the full stiff hole. I ran a best of 68.6; a 2013 Shelby SVT pack ran a 67.5, and a stock 2008 vet went 69.3. Still learning.