Actually that should have had the opposite effect, I assume you mean "soften" the rebound? This is also one of the traps you can set for yourself, if you've done everything to "stick" the back half of the car, it's time to free up the front, because it sounds like the front is stuck like glue, once the car gets to a neutral balance, then work on both ends. A short term solution to this (or to help find out what is going on) is to jack up front air pressure for a few laps. Also, the front shocks should be "soft" in compression. As an example....
Here's a fox body that I've built, it has an IRS in it, cage, k-member and coil overs, as you can see the car "pushes" in the center of the corner.....
Here's anothe rpic of the car in a different corner, still pushing at center...
This is a pic immediatley following pisc #1, note that not only did I miss the cone but that once the car started to transition to a rearward weight, the front of the car gained traction and the car launched....
Here's another pic of the car accelerating, notice how high the front end is as the car accelerates.
So how do you fix this? (we can't run adjustable shocks, but you can) the best way would be to take some dive out of the car in the center of the corner by raising compression. In a short, fast autocross course the car is dumping it's weight on that right front "en mass" slowing this rate of dumping would most likely allow the car's suspension to handle the increasing weight and make the car turn. If that didn't work, I would up the front spring rate and then loosen the rear of the car. Increasing the front bar size would just "unhook" the front end even worse although it might help a little, you would see less overall grip, which I don't want, the front of the car is actually sticking too good...then pushing. In your case, I believe the same thing is happening to the rear of your car, basically , you are overwhelming the suspension, you've already done bar and shock adjustments so now it's time to loosen the front in order to let the car manage it's rear bite.