I have been thru at least 8 sets of EBC Yellows and this is normal.
With so much grip, its easy to use the brakes to their maximum ability and get them hot as hell and then see a little bit of smoke when you come into the pits.
Then, after a track day, they will not grip as well as they did at the track. After a little bit of driving and a little heat, they will return to normal.
The pads are well cooked but not beyond their intended design so you should be good for your next event. Just be sure to regularly change the brake fluid (using a high temp fluid) since its getting cooked too.
Take a look at my worn pads and fluid:
Spec Boxster Build - New Brakes