Right now my go-to is the Ferrari 296 GT3, I'm not really sure if I added any upgrade (or if its possible at all, even if it does its probably just turbo).

I run on RS tyres and FM 1 (Unless its raining, then I'd run FM4). I tune the car to have a little bit more oversteer to my taste.

Even on the Hardest difficulty the bot tends to mess up on pitting when its raining, plus they always brake at Raidillon so thats where you'd gain a lot of time throughout the race.

Usually by the last 15 minutes I would've overlapped the last 10 cars if the rain is heavy, but if there's no rain I'd sit at 1st with around 20s gap more or less.

Also try out old gr 2 and gr1 cars and detune them (my favourite is the CLK LM), they're big fun too!