First post here, so don't flame the no0b until I do something REALLY dumb. Anyhoo:
Understeer is always going to be a problem on a car with a center diff. I was driving a '91 BMW 325iX (AWD) and had terrible understeer problems until I put 185s on the rears. That brought the car in a little tighter, but it was still tough to get the trunk end to rotate until I put in a 26mm rear sway bar (+9mmover stock). Then everything swung around the way I wanted it to.
It really came into its own in the snow: I could put it into a drift at any point I wanted, hold the drift as long as I needed, and exit pointing exactly where I wanted it to. Of course, it was a BMW, so it was a pretty expensive snowplow.
I just bought a 1989 Loyale Turbo. We'll see what happens--this forum has given me a bunch of ideas already!