Faa Ir
That is exactlly what I intend to do at the appropriate point in time that I decide to shell out for a JAA medical and revalidate my CAA PPL. For the moment I am happy and able to move around solely on my FAA PPL and $50 medicals.
Assuming you got it attached to your PPL SEL I believe you would have to retest, rather than retrain, the IR anyway for class of aeroplane you are flying (ME for instance) and Licence to which it is added i.e. CPL when you get to that point. Retest if reqd is simple as it requires 3 hours instrument instruction in the preceeding 90 days and an FAA instructors endorsement to sit the test.
The training for the initial IR is more than the IMC :- 40 hours instrument or simulated instrument time, inc 15 hours instruction and 3 of those in the last 90 days. But at US prices I don't think the difference is significant.