The differences between FAA and JAA IRs are irrelevant. The FAA have lower tolerances in some areas, and higher tolerances in other areas - partial panel for example. The format of the FAA IR is "on the fly" - as soon as I took off, the examiner slapped a post-it note over the AI and DI and most of the test was partial panel.
Anyway, I'd advise anyone to do the IMCr now as it will be issued as an IR(R) on your EASA license. I am sure there will be an interesting conversion route to the EIR too, meaning that you could pretty well end up with FULL IR privileges in the UK (Fly IFR in class A and shoot approaches - due to the EIR and IR(R) ) as well as enroute IFR throughout the world.
This is cool, and as much as most people need. It'd make an interesting court case, someone with an FAA IR in an N reg plane with EIR and IR(R) shooting an approach in Euroland....I wonder how that would pan out.....!