Not sure I follow that Will. Why would you do a ME CPL then a SE add on? Most people I know did SE IR, SE CPL, ME conversion finishing in a assymetric ILS which gives you (in FAA land) a ME CPL/IR. I did both CPL and IR on ME for similar reasons to the ones you suggest.
I agree doing ME CPL makes conversion in UK easy, but as for IR, sadly not from my experience. You would think holding a FAA CPL/IR and being able to fly a 'N' Reg aircraft anywhere in the world would make the JAA IR conversion a doddle, but most will tell you the FAA and JAA IR are very different (mainly due to use of NDBs, en route segments, and strict comms in latter and that you can fly the same test route over and over in the US). Seems to me that the FAA see the IR as a safety issue and encourage every PPL holder to complete the IR, whereas the IR in the UK is seen as a professional qualification and is deliberately tough in the theory that if you can fly a ME aircraft in IFR single pilot then you can definitely fly in a multi-crew, fully automated, environment. This makes the courses very different beasts.
No one I know who has trained in the US has completed the IR conversion in the minimum of 15 hours: if you expect 25 hours conversion plus the 45ish in the US for the IR then the 50 hour (post CPL) IR course in the UK becomes more attractive.