Derek,
I did my IR in the UK in 15 hours (I was ready in 12). The main differences/difficulties were :
- R/T a little different. In 5 hours you get it figured out.
- Use of ICAO flight plans instead of FAA
- The routes are MUCH shorter
- Relearn NDB approaches (never used them in the US before)
- Altimeter setting procedures
- Much stronger winds that I was used to.
- Tighter standards
The good deal would be to go to a FAA flight school that has dual-rated FAA/JAA instructors who can teach you the differences from the very beginning.
good luck,