Remember to do an FAA CFI you need an FAA CPL and FAA IR. That said if you also have a JAA FI and JAA CPL then I think this makes you quite employable in the UK, as like myself FAA licence holders have to do a BFR with a CFI every 2 years (can be done in G reg in UK). This brings the possibility of bringing new blood into your FTO. Just my thoughts on it - complex, but could be worth it.