Don't do an FAA IR and then expect to convert it to JAA in the 15hrs. This is the MINIMUM requirement. Unless you're flying IFR most of the time you will not get it done in 15hrs.
The 15hr conversion is usually allmost 50% of the cost of a whole JAA 55hr course. Remember extra hours in the air are about £350 per hr!! (False economy)
Inst flying in the states is very different to UK.
Do everything in the US, APART from the IR!