Haven't tried it, but that doesn't sound good. Soft tires, minimum pressure, in an LMP900 car...if it's going to have a handling fault, it should understeer, not oversteer. It's a wierd sensation that I get with a lot of race cars on slicks in this game...lots of oversteer, very unstable rear end - I've never driven a GT3 car, but my understanding has always been that majority of the time, you set up a racecar (especially a customer car) to be just a shade on the understeer side, to give the driver confidence to lean on the car.
Every car I've tuned in the game, I've had to stiffen the front, soften the rear, and at least double, if not triple or quadruple the locking power of the diff. Only cars I've been able to drive with confidence on the default setups in the M6 GT3 and the Enzo. This could all be down to my personal style and preference in handling, but I'm starting to question even that.