My FAA IR writeup from 2005/2006 is
here 
It takes a while... FAA PPL, FAA IR, medical, 2 exams, training, 2 checkrides, living in the USA for a few weeks (possible but very messy in Europe). As I said, OK if your time is worth little.
It is immensely frustrating that the JAA IR bar has been set so high and has so many arbitrary steps which are no more than purely in-your-face business creation e.g. the 50hrs all done at an FTO, with no credit for previous training (other than another ICAO IR) but I can't see the conversion route being efficient unless you do want 2 sets of papers.
If OTOH you are mobile and want to keep the options open, then get both
