A week ago I did the same thing.... Started out with a Fiat 500 on the nurburg. 24 h course, 2 laps and right then and there I found the same thing as the rest you. It was a completely different experience. Well, the Fiat 500 race was just for the fun of it, so I went for the stronger cars, the 200-500 bhp cars and what surprised me was how well I actually knew the tracks, their breaking points and when to change gear. I've relied so much on the HUD to help me out, that I guess I've missed out on the feel of the car.
Not anymore.... This is the best thing ever, driving without the HUD. Also I've always raced with the skid recovery on, so while I was in the progress of making my racing a bit more authentic, I removed that too, so now it's all stock, tyres and so on. Even bette than the best thing ever. I was bored with the game up until this happend, now it just feels like a new game.
I can only recommend it. People will be surprised how well they actually know the cars and the tracks when driving without the HUD. Start out with the slower cars, to get the feel and then go for the faster cars eventually.