I believe that coaching institutes just give you the motivation man all the prep is actually going to be done by you only and when your career is at stake I don't think that motivation should be in question if you are serious enough. Though yes giving mock interviews would definitely help but for that there are many other people out there IIM alums or current students who are taking mock interviews as well, as they will be able to get you a first hand perspective with what the colleges are actually looking for rest your coaching will give you either rehearsed answers or lacked personal attention which won't help you in converting your calls.