Well i find with the 599 its got so much hp that the main problem is the back end going out with even a slight miscalculation on the throttle, and near impossible to correct, which is pretty spot on to real life with all aids turned off.
But my suspension is bone stock to since i knew better than to mod it up a great deal. Understeer in these cars usually means your braking too late or braking too hard into a turn, straight line breaking ftw in any ferrari or super car pretty much, if anything tapping the breaks into a turn will transfer weight to the front and give your turn in a bit more bite, but slamming them on will lead nowhere good.
Plus the custom suspensions are all setup generic, they are not tuned to the car when you slap them on so more often than not they will make any car worse unless you know how to adjust it for the cars tendencies and your driving style.