These pads saw 7 20-minute sessions at Thunderhill. Should I expect to see cracks in the pad material? They still have about half their original thickness left - can they be used for another track event?

I have a 2000 S4 with Stock calipers, Stoptech stage 2 slotted rotors, had fresh Motul RBF 600, and these are the Stoptech "club race" pads.

<img src="http://pictureposter.audiworld.com/940/cracked_race_pads.jpg">

...but be careful as you get worn down closer to the backers. Have spare pads.

uneven, as in one side has worn down faster than the other side or it wears on an angle (neither will happen to you with ST). Or chunks of pad breaking off the edges.

Your rotors will crack bigtime as well. This is okay as long as the cracks aren't deep and don't get to the edge of the rotor.

And if you plan to continue tracking your S4, shell out for a big brake kit. Pads are cheaper, last longer, and work better on the track.