I managed a 1:29.5 with the default settings before I started tweaking, then was consistently about 0.5s faster and did manage a 1:28.8 with my new setup. Basically all I did was lower the ride height and reduce the anti-roll bars to allow the car to settle into the corner better, pretty noticeable difference for me.
Give it a try.
Drive Monza like Jackie explains here (though he is talking about Monaco I think this really applies to this era of car in general), this is how I drove the car to gold as best that I could manage.
This is my lap