finally I had a chance to track the car after the rebuilt of the diff with the new friction plates treated by WPC
(cusco rs 1.5way 80% lock)

the difference is outstanding...
track performance:
no more push understeer. period.
I feel the diff is there, no wheel spinning going out of a corner, the rear end feels stable in medium/hard braking, the traction is there.
and most important.. no more fluid boiling and spilling outside.
my mechanic put a fuel filter on the hose to check the spilling... not a drop, before I was giving a nice oil shower to the exhaust. now nothing.

I have to adjust my suspension setup because from before that the car has some understeer on exit, now is on the oversteer side. not impossible, but I need to soften the rear for sure.

on the road.
before, after one track day... I had to change oil right away because was impossible to drive. chattering all the time also for wide corners.. now, it's quiet.. some chattering in the parking lot or sharp 90 degrees turn.. otherwise.. quietness.