Genius
You do require a FAA medical for your FAA IR flight test. You will not be given a stand alone certificate as the Instrument is a rating and not a license so it will be added onto your FAA private which is valid only and based on your JAA PPL. It will however note on the back under the IR portion "U.S. test passed"
Hope this helps.
PS, It seems from the above post that there will still be confusion, I think it is going to depend on the DE as I did mine and required it, I also continued on to do my CFMEII and instructed foreign students there and they all required it as it was an addition of a rating.