Back in July this year I had an FAA IR added to my FAA PPL (61.75 Restricted version). Although this was only for four days before I passed the FAA CPL.
If you obtained your restricted FAA PPL BEFORE the new verification procedure then you will need to go through the verification procedure to obtain the "Letter of Auhentication" from the FAA.
Although you need to specify an FSDO as you have an FAA PPL issued under this part already you will not need to visit them.
All that is required on your FAA IR Checkride with the Designated Examiner is the letter of authentication, the FAA PPL and the JAA/CAA PPL that it is based upon. As well as all the other paperwork and checkride fee that is! ;-)
When you pass the IR you will be issued a new temporary airmans certificate with "Instrument Airplane" under the ratings section. You will also have the proviso:
Instrument Airplane (US Test Passed)
This certificate is valid on the basis of and when accompanied by United Kingdoms CAA Licence number PP/xxxxxx/A.
The designated examiner will then send all of the paperwork off to the FAA *including* the letter of authentication. So the FAA know your JAA/CAA licence has been verified as valid.
If you go on to do the FAA CPL then the all of the restrictions will be removed and the CPL will be a standalone certificate. But you will need an FAA medical if you do this. The letter of authenticity is only required on the first checkride, the second (CPL in my case) didn't require the letter of authenticity.
In order to take the FAA CPL checkride you do NOT need a full unrestricted FAA PPL, the restricted one will be just fine.
Incidentially I had a suprise when I opened the letter which contained the certificate from Oklahoma... the FAA certificates are no longer paper! They are credit card style with nice pictures of a Boeing, Wright Brothers etc on them.