It's possible to win on Sardegna with a Corvette without a tune. The car is so fast that it's possible to catch the Viper, even if you need to do 3 stops. No need to change the Fuel Map, just go as hard as you can.
Another good set of stock options are the BMW Z4, BMW M6, AMG GT and the AMG SLS. All of them can complete the race with 2 stops, so you will get 1st place when the Viper needs to do it's last pitstop. I don't recommend using FM with them. Just try to race as smooth as you can and you will win.
You can use Turbos on both BMWs, but I'm against it. What you gain in speed, you lose in fuel efficiency, so you will pit at the same lap as the Viper. Basically, it's the same strategy as if you were driving the Corvette: just go as fast as you can.
Good fuel efficiency cars, like the Renault R.S or Pegeout RCZ, have a hard time because they're way too slow compared to the Viper or the AMG GT, but, since their efficiency is so good, you can push them to a 1-stop race using FM6. It's a tight victory because you can't make mistakes. Unless you're playing in easy or normal, don't expect to pass the Viper while driving these cars. That AI Viper can reach 300 km/h at the end of both long straights, while the R.S/RCZ can only reach 260/270. It's hard, but it's fun!
The Subaru GT300 makes Sardegna easy. Don't even worry about pitstop strategy, just go for it and like you said, it's the same thing with any detuned GT500 cars.
As for Spa, I've tried this race with all my current Gr.3 and I really didn't had any difficulties. The AI, even on hard, is not good. They often forget to change to wet tires or they don't have enough fuel for the last laps. I honestly think you can bring your favorite Gr.3 car and just enjoy the race.
In all Gr.3 races I always use Racing Hards. Have yet to find a reason to race with RM or RS.
if you're looking to grind on easy/normal difficulty, any 730pp setup done by the lovely tuners on this forum is more than enough to demolish those races.