I did my own math. The four teams in the AL East besides the Sox have scored 1245. The four teams in west have scored 1153. That's less than 8%.
The Sox play, what 18 games against other teams in the East? so, 72 total in the East. That's less than half the year. So the difference in runs would be around half as well, for the sake of argument, lets call it 4% total. So yes, being in the East affects your pitching, but if you're playing on one of the Sox/Yankees it doesn't affect it as much as you seem to suggest.