Well if you are going to break it down year by year at least use something like xFIP that is somewhat stable and not so heavily luck based. Bush - 4.54, 4.45, 3.76, 4.36, 4.67, 4.79, .4.97. His GB% has been pretty steady, his K/BB has been pretty steady, he has consistently given up around 12% HR/FB. While his ERA looks good this year his skills are worse than they have been in previous years, unless something changes expect to see that ERA jump up a lot very soon. He is off to the worst season of his career so far this year, it just isn't showing in the ERA. This would support your suggestion btw but I still don't think Maddux is a huge part of the reason, I just think the league had adjusted to him in 2007 and he isn't the same guy since the injury.