I also have an FAA IR. I have yet to jump on the JAR bus. It is worth noting that the FAA system is as good as if not better than its European equivalent in terms of efficiency and functionality.
I personally am about to do the FAA Comm. This is essential for me since I need to fly N-reg aircraft. However, these are short term prospects and I will have to do JAR if iI am to get a reasonable position in the UK.
I hope there won't be any major problems with doing this, but as ever, politics plays a big part in aviation.
Cheers.