Sigh...FAA bashing again...
I agree with Lightning_Boy. There's a lot of unfair dispersions cast on the FAA IR (and the FAA licences), if you go to a decent school in the USA the IR will be bloody hard.
You don't "convert" an FAA IR to a JAA one. Having an FAA IR cuts the minimum training hours down to 20 (I think, please correct me if I am wrong). That's all.
Having an FAA IR will no doubt make the JAA IR easier and probably slightly cheaper.
Of course you are not going to get a North Sea job with just an FAA IR. The North Sea is in Europe. The employers have the luxury of asking for what qualifications/hours they want from people, as there are tons of people who want to fly the rigs, but only about 10 jobs per year. If more people paid for their S-61 type rating, then the employers would also ask for that too.
I have spoken to people who have done both FAA IR and the JAA IR, they all say that the flying/procedures is pretty much the same, except in the JAR IR you have to learn how to fly a turbine with a stabilisation/autopilot system.