What you say is true, and an FAA IR holder expecting to convert in 15 hours is unlikely. I also agree there have been complaints about some of the "CAA approved" IR schools essentially training IR students to US procedures and tolerances and expecting them to come up to scratch in 20 hours or so in UK.
However there is a new breed of training organisations opening in the USA, operated by JAR people. There is no need to be CAA approved as all flight tests are carried out in the UK. Becasue these places are run by JAA people who know the conversion system, and the training is carried out to JAA procedures and tolerances even though under the FAA system, then conversion is a straight forward matter.
The key to going down this route is to ensure you ARE trained to UK procedures and not just let loose under the FAA system (which is essentially what a lot of the CAA approved IR schools do). The two systems are substantially different, I'm not saying one is better than the other, just different.
I know one school who advertise that they can take a PPL who has 150 hrs or so to JAA ATPL (inc FAA CPL/IR(ME) ) for around £12,000. Assuming some of the 150hrs have been done in the UK then your point about the CPL conversion may not apply. If they do really know and understand the conversion process then it seems a reasonable route to take. I know one guy who went this route with this particular school and he converted first time, CPL and IR in the minimums at a place in Scotland.
Anyway, its something worth considering before anyone chucks £40,000 into the pot.
Cheers
EA