There are very few sports cars that be dominant on both the short course and the track. If you watch Top Gear and all of the Stig runs the Nissan GT-R is one of the few cars that can do both. The electronics and power transmission is just getting better and better on new cars every year. Last year Car and Driver magazine (I think) pitted the newest 370Z vs. the latest Mitsu. Evo from the showroom. The Nissan looked a lot faster on paper, 200lbs. lighter, obviously lower and sleeker than the four-door sedan, guess who won though on the track course though--the Evo.
Even in a hopped up 1303 with as much power to weight ratio, I still wouldn't feel as safe in an equally fast 911 above 100mph, but that's me. Now, if you're talking short course events like autocross, a bug can be made just as competitive. Difficult to do with a 40 year old car, but it can be done. New cars have the advantage because everything is still so solid. Power is not the most important thing on a short course though. An '89 Honda CRX with all of 100 hp stock, but a completely dialed-in suspension and tires will still pull some of the fastest raw times of the day, as fast as a Lotus Elise or S2000 or Z06. It's how much momentum you can carry and how nimble you can be around a course.
I took a picture of my bug at last Saturday's autocross, it won't upload here, but even with the car lowered and next to all of the other usual suspects my roofline was the highest (except for a Toyota Matrix out there). I attract a lot of attention and run a basically stock engine. I don't dominate my class but my engine/trans are not even close to what the limits allow. I have a lot of fun and keep up with everyone else. If my car is slower, it's only a little slower compared to cars which easily have 2-5 times as much power
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