If you're going to the States to get a FAA CPL are you aware that you need 100hrs PIC whereas in the JAA system you need 50hrs? This could affect your deicision depending on how many hours you have at the moment. If you're going for a JAA CPL then you are limited to a certain number of schools and helicopter types when compared to the UK.
Assuming you have 100hrs at the moment (100+150= 250hrs min requirement FIC) I would be tempted to fly the hours for the CPL here and then go to the States, once you have done the CPL course, to top up your hours for the FIC. BUT remember that you can get an R22 for about £140ph in the UK if you look hard enough thus avoiding any potential 'stigma' issues.
A large number of people consider the FAA IR a waste of time. Moreover if you're looking for a JAA IR, I've heard the flight times to convert FAA to JAA vary wildly from the bare minimum of 15 hrs, to the full 55hrs. So the IR could indeed be wasted in that instance.
Bobby