I read this thread, and also another one from last year... Damn, the 3rd round at Monza is annoying (but challenging).
Because I'm often play Gr3 and Gr4 racers with proper RH/RS racing tyres, so I didn't want to run SH/SS on these racing machines. I honestly don't know if the 458 Gr.3 can give an easy win or not.
I tried to use the F40, either 100% stock or detuned to N400 as some guys suggested. The best I could do was P2, with 1-2s behind the "rabbit" 330 P4. My average laptime around 1m59s, and total race time around 12 minutes for 6 laps obviously.
End up I spent some lap practising time-attack with a 100% stock 458 (N600 I remember). With probably 20 laps of practice, I could consistently do 1'58'100-500. And with that pace, I manage to pass the 330 P4 on lap 4 as it ran wide on the T1 chicane. Thanks to the rubberband effects, the 330 actually caught me up on Lap 6 -- but ran wide again on the Ascari chicane again. Eventually I won by 1.8s -- and total race time only 11m48s.
I think you have to be good and consistent enough at Monza in order to win the race with a 458 or F40. I reckon total race time has to be under 12mins for 6 laps to get a realistic chance of winning. Hope this helps guys!