If your line isn't opening holes, why do you want a bigger, slower RB trying to get through those non-holes? You think it will get easier to block between the tackles when the defense doesn't have to worry about any runs outside the hashmarks?
If you want to upgrade Maroney, at least get a threat that keeps safeties on their heels and linebackers from crashing inside.
BTW, Maroney is 25 yo. Shonn Greene (the present and future of the Jets) is 6 months younger. Methinks there might be time for young Laurence to yet reach his potential.
Gerhart wouldn't be an upgrade for Maroney. He'd be an upgrade for Taylor or Morris.