Well, to give an update: since the differential settings seem to have absolutely no effect on my evo x's understeer (despite a 90% rear torque bias), I decided to see if anything in the suspension settings would enact some kind of change. So, I stiffened the rear more, and still nothing. Back and forth I went, from practice to tuning, and then, I started to notice a trend - no matter what I did, I could not tell a difference in how the car drove!
It was at this point that I began to develop a sinister theory, so to prove it, I decided to run absurd settings that would be a disaster in real life - I set the suspension to MAX stiffness on EVERYTHING in the rear, the ABSOLUTE MINIMUM on EVERYTHING in the front, MAX toe out in the rear, and... (drum roll)... NOTHING! The car drives EXACTLY THE SAME! I set the front diff to full open and the rear to full lock - nothing. I reverse it - nothing. THE TUNING DOESN'T WORK IN THIS GAME! It's just there for show! Absolutely disgraceful, this game.