Ah! ColinB,
You get around also. Not too much activity on this site is there?
Looking at the old evaporator (as you might have seen elsewhere) I don't think this flux deposit would have any effect on the operation of the the ac as it is external. As long as when originally fitted the system was flushed out with nitrogen. Speaking with the Service Manager he could not give an opinion, though that is par for the course . May be a bit messy until everything has blown through, but that is all. It appeared the deposits were much worse for some people, but it is only cosmetic and a wipe up with a cloth is all that would be required. On the paperwork from Mazda, Japan there wasn't any mention of any effects of the flux dust, the replacement is something to be asked for by the owner. So if they are not aware ( not heard about it on these websites) then the work will not be volunteered by Mazda, unlike a safety or environmental change.
One advantage to me was a free top up of freon.
There are no funny noises from the dash after two motorway journeys and none on local potholed roads.