"Has anyone done a Mikuni carb conversion such as offered by Hitchcock’s on a Euro IV C5" No, but I did an Amal carb conversion kit on a 2013 B5, and couldn't be happier with it. The bike runs 100 times better, and that is not an exaggeration. It FEELS like it has a lot more power, but that may be because the carb fattened up the mixture a lot. No more sputtering and hesitation. No more wheezing along at 55 mph. It now pulls hard, sounds much better, and has instant throttle response.
I am very familiar with Mikuni carburetors, but only on Japanese bikes. I don't know how one would work on a RE. Are you talking about a CV carb? Also the Euro IV bikes may have a completely different EFI system than the first gen EFI bike.
If Hitchcock's says you will not lose power, I would tend to believe them. I've dealt with Hitchcock's many times over the years, and always been satisfied with their service. The cost of shipping from the UK to the U.S. is another matter, but that's not their fault. I would actually be willing to give up some power just to get rid of the EFI, but I seriously doubt that is the case.