Here are some numbers that help support what I believe. This is Mark Buehrle's numbers on different days of rest.
4 Days(standard) 18 ER 18 1/3IP 6BB 15K 2HRA and a 8.83 ERA
5 Days 1ER 15IP 2BB 8K 0HRA and a 0.60 ERA
6 Days 3ER 6IP 2BB 5K and a 4.50 ERA
7 Days 2ER 7IP 1BB 4K and a 2.57 ERA
Now, it's still a small sample size since it's only May 13th, but when Buehrle gets more than 4 days of rest, his number stand at 6ER 28IP 5BB 17K and a 1.93 ERA. 3 Starts so far this year on 4 days rest and 4 starts on more. I'm going to keep logging this for the rest of the season, or until he goes on the DL, whichever comes first.