i agree with rynman. MR's get less K's than starters along with IP. So by replacing starters with MR's, W's, K's, and IP can go down. Another thing is that middle relievers can have bad outings just like any other pitcher. Except when they have a bad outing they get pulled after an inning (maybe less) while starters tend to be kept in for more innings. Therefore, a MR's ERA and Whip can bolster more significantly than a starter's after one bad outing.
you'd be better off replacing bad closers with top notch MR's and punt saves.
"24 hours in a day, 24 beers in a case. Coincidence? I think not."