I sort of disagree with all the 'waste of time/money'.
If you already have a JAA and FAA CPL IR, and a JAA FIC, then you need
two written tests - one easy, one similar to the CPL test; buy the books and software; you don't need any kind of course completion endorsement to take these - you can self-certify you are ready
training only 'as required' by an FAA instructor to take the test
Although the FAA CFI is demanding, it is only a relatively modest increment of time and cost compared to everything you've done to get the JAA FIC and the pair of CPL/IRs and you should be well prepared for it.
Not many Instructors are JAA and FAA dual rated as instructors, so I think it is a useful qualification to have.