Sardegna can be done on Normal and Hard, but you have to get 5 laps minimum from a tank and run I believe 1:39 averages in order to win/be close to a win on hard, with some 1:36's or better mixed in as well. In those cases you'll probably hang around 8-4th place until lap 4, then jump up a spot, and then you need to bump the fuel mapping up a bit and close the gap as much as possible, but I think everyone ahead of you will be pitting after 5 as well. You really need to get 6 laps on hard if you want to win with a comfortable margin of 10sec or more. That way you pit after 6/12, get a full tank, and run FM3/4 when needed to close any gaps.
McLaren F1 can do it on hard. I've done it with the F50 but that was before the physics change in 1.23 when that was still the fastest road car in its class; haven't really tried since. Porsche GT1 Strassen, but that's essentially a race car without the #race tag. Key is to drop the Power Restrictor and keep the ECU high as you want the torque to peak as early as possible and you have to do some extreme short-shifting in some of these cars, like as low as 4000rpm. You have to tune around these constraints and the low front downforce that will limit the speed you can carry through corners. I like a \/0.15 or higher front toe with a low front/high rear ride height so I can freely roll around those 90 degree corners without being on the gas, which takes some practice to get the brake-release right so that you carry good speed while rolling around corners without slamming into barriers & have a good launch angle. It's different for every car, though the principle is the same.
Essentially you have to setup cars and learn a driving style that's probably totally different than anything you're used to just to run road cars at Sardegna on Hard/Normal. I still do it from time to time, but I'm so burnt out on the game/the same three races I usually just put it on easy and run what I want to try to have fun.
@shadow0460 Try to get the 917k before it leaves... It's a really great car that is good for some of these events. Only bad thing is the downforce is lower than many super cars, but its 50+ years old, so understandable. Either that or the F1... Both are awesome, F1 is a little more versatile as it can win all three of the races that matter (though the 917k can too if you run Sardegna on easy). Boy does it hurt buying those cars though!
