What piston single will come down the ILS at 100kts?
What piston single can fly IMC in training legally?
What piston single has SAS or an autopilot so you can manage taking down the weather etc.
What single will let you practise an IMC engine failure on the go-around and on finals during either an NDB approach or an ILS approach?
For flying a twin, which is what you need for real world IR then these things must be practised in training, it becomes a handful very quickly up there.
I know of one person who did FAA IR and FAA IRI (CFII) and still decided to do the entire JAA course as he needed to. Also another who had been using his FAA IR on twin but required significantly more than anticipated to pass JAA test. When it comes to something as dangerous as IMC IR those looking to make it as easy and cheap as possible should stop and think. I did my research beforehand too: Bristow North Sea said they would not employ me with FAA IR and try to convert it as the last time they tried their students had to do the full SE IR course at Norwich to make the grade. CHC had to convert some FAA IR's recently and ended up letting some people go as they couldn't manage to make it either, so no go there. Finally Bond just said "we will not give you a job if you have an FAA IR" and also said they would not attempt to convert it. So regardless of opinions if you want a job like that, a great IR breeding ground as you could still kill yourself if you blast off onshore straight away, then do as they ask.
Having done the full JAA course I would reccomend it, its tough for a reason. IMHO
