You're not going to like this.
Whenever someone says there's still "understeer" at the limit, it usually raises a flag in my head that says "said poster has no idea what understeer is."
Figure out WHERE this so called understeer is coming from "at the limit." Meaning if it's at corner entry or corner exit, will come in very handy in how it's actually fixed. If you can't tell me where it's coming from there's no point trying to pin-point a specific component to fix it.
And to be completely BLUNT, A fix for a single corner isn't going to work for all the corners on a course. If you're telling me the car is understeering at the exit of a particular turn on a certain track, you still MUST weight the entire track as an overall solution and fix/tune to neutralize the chassis for the entire track. Saying "my car still understeers" is absolutely meaningless.
Take Buttonwillow, for example. I can tell you that my car as is currently set up will understeer INTO Buttonhook, turn 2 in the full-course configuration going clockwise. I can certainly stiffen up the rear to give up some grip in the back to force it to go neutral for turn 2, but the end result is that the car will have a tendency to OVERSTEER at Star Mazda, which leads to the FASTEST section of the track. Fixing for "understeer" upon corner entry at turn 2 is pointless. The speed I gain there will be promptly lost back at Star Mazda since at the end of the next complex you're looking at speeds in excess of 120mph, while at best, exiting Buttonhook leads to a short straight that sees the top of 3rd gear (about 90mph). So if it makes it harder to navigate Star Mazda at high speed consistently, I'll be happy to deal with corner entry understeer for Buttonhook by giving up some more speed going into it.
