concerning FAA IR, you will need at least 40 hours on a multiengine to convert your JAA IR. it's harder to fly in the US than in the UK, more traffic, more procedures, ILS back track, ndb, circle to land, arc DME, lot of radio work, plenty of SID/STAR,NDB hold, ...big airports (LAS,STL,JFK,...)
it is why the FAA ask you to pass the IR written and check ride.
you will take a bath in the real aviation world!!