...all you'd need do is the JAA IR theory subjects...
.. but isn't it the the theory that's the problem with getting the JAA IR. From what I know, and as an FAA IR holder, there are some differences in the flying test but nothing that any reasonable person could object to. It's the differences in the theory requirements that make the FAA IR so attractive to get in the first place, and are the major barrier to people who want to fly IR in Europe.
Before anyone points out the differences in renewal requirements (annual test vs experience), I know about that, but IMHO that is less of a disincentive to get the JAA IR than the initial hurdle of the theory exams.