Doug's CTS-V wagon article had me thinking how safe it is to let the car do the shifting for you when you have a bunch of power under your right foot.
I've never driven an automatic shifted car with more than 250whp, but I have owned plenty of slushboxes, but they've been relatively low on hp. You step on the pedal hard and it drops 1 or 2 gears and goes—not really too dramatic or anything, might get a chirp if you're lucky. That said, all the autos I've had have been in less than exemplary cars
One of my cars is a new mustang optioned with a manual 6 speed that makes about 360 or so at the wheels. Not amazing, but not bad. I have been able to get it pretty squirrely if I drop it in too low of a gear and punch it—traction control be damned. Over time I've learned which gear is appropriate for which speed I intend to go (for the most part). I don't need to trust the car to guess what I'll be doing.
This is especially crucial around a corner...If the car chooses the gear for you, is it smart enough to know what is too low of a gear for your throttle position to avoid undesired wheel spin? Say you have no flappy paddles, just the car reading that you've tromped the peddle.
My only experience around a track in anything like this was a v10 R8, and you can bet I was shifting myself.