I'm not trying to argue with you since each person's perspective of "negligible difference" is different.
But I've run the 400lbs and 500lbs front springs, before settling for the 450lbs, and the street comfort level between the 400lbs and 500lbs is quite noticible. The 500lbs ride quite a bit harsher, especially since Seattle roads are crap. I actually wanted to keep the 500lbs front springs because the car just handled a lot better with a lot less body roll and turn-in felt sharper; I stiffened up the reb/comp settings with the 400lbs springs and they just didnt feel the same. I went back and forth with the 400lbs and 500lbs springs a couple of times before saying screw it I'll just spend another $220 for the 450lbs. The 450lbs springs to me actually feel much closer to 400lbs than 500lbs in terms of ride comfort and handling.