Mark Buehrle's career ERA in the 7th is still 4.23. Verlander's is 2.40, but that seems to be an anomaly since his 6th inning ERA is 3.73 and 8th inning 5.14. Bronson Arroyo: 3.78. So the first three I've looked up, two soft-tossers and one premium arm, don't seem to be as rapidly declining in effectiveness after the 6th as Dickey.
This does make some sense: hitters will get used to the knuckle eventually, while Buehrle and Arroyo are very good at mixing things up and don't rely on velocity. But then Jered Weaver declines from 3.47 to 4.65 from the 6th to 7th innings. Possibly because hitters get used to his wacky delivery? I'd like to see average stats for the whole league per inning but I'm not a baseball-reference expert.