If you complete your IR in a S/E aircraft you will have to complete a minimum further 5 hours M/E IR training plus another complete IR test it is much simpler and cheaper to just do the IR on the multi in the first place. In fact, the most sensible option is to do the CPL on the Multi, it works out cheaper in the long run. Also the 15 hour conversion is a bare minimum and most people will need more unless they have significant IFR experience.