That split second when you've clutching and have only one hand on the wheel can cost you, it's probably not nearly as important with an older car where you're compelled to leave a larger margin of safety but if you're going to really push a car with awesome handling and braking then full manual shifting starts to become the weak link.

One observation I've made is that when I drive high horsepower cars w stickshifts (drove one w 600hp dyno'ed, another with about 800, also dyno'ed), on the street, I end up being FAR slower, careful and deliberate, particularly for ensuring I'm in the correct gear, and release the clutch more slowly, than in my mostly stock Nissan sportscar. Why? Because getting the wrong gear could be VERY expensive, and also potential for loss of control too (rear tires break traction). IMO this is part of the reason today's Ferraris and Lamborghinis, with more than 500hp of instant power, are being bought with semi trannies: stay in control, less chance of damaging the drivetrain, AND better performance.

But ya dont gotta like it! Heck, if I won the lottery, I'd get a Gallardo or F430 with the semi, AND buy the purest Porsche, the GT3 normally aspirated and stick/clutch! Hey, dreaming's free right?