That's currently my method for finding what tire pressure to go with for the front. My tires seem to like 33psi. I'm gonna experiment with low pressures in the back.You need to learn how to chalk your tires so you can learn where the edge of your grip is between runs. Also you can't just set it and forget it, as temps go up so will your psi. This past weekend at autocross I had to bleed 2-3psi from both front tires and 1-2psi in the rear 2 between each run to keep them in the optimum window of grip. My fronts are set to 39psi and my rear to 35psi, Firestone Firehawk Indy 500's for tires. Sounds like you need a rear swaybar upgrade(if you don't already) to help combat that understeer because I never felt that I was plowing unless I made a mistake on entry which I correct for with more braking when such mistakes happen.
I currently have a progress rsb but am looking to get the stiffer JBR one or up my spring rate.