You will need to get clearence before even being able to fly in the US. You can get a FAA PPL off your JAA licence, but will then need to get a FAA CPL, which will require flight and ground training. Don't think you need the IR in order to become a CFI (only if you want the CFII) Then a CFI course, flight training and two ground exams as well as one oral exam (dont want to put you off but my CPL oral exam was about 4 hours and my CFI oral lasted about 7 hours (with coffee breaks inbetween). Worth doing the IR and CFII as this gets you lots more work. On return to the UK, you can do a reduced course for the JAA FI which enables you to do 30 hours of ground and 15 hours of flight before the LST. This is the way I did it, however I'm a fling wing guy, but I'm guessing its the same for the planks!
Hope this helps
LB
