I decided to just remove the stock tips and get them sand blasted and power coated, and see how that went. Cost was GBP £50 but I'm told that was pricey by others who've also had it done. Guess I should've turned up at the workshop in my 1.4L runabout and not a $55K M235i - I never learn. Here's how they came back from the workshop:
They look good and have survived the last winter fine. Main downside is that whilst the coating is great, once refitted, the inside of the tailpipe is still visible and the usual dull grey colour.
So more of a case of Does the Job compared to the corroding stock tips, but not necessarily a high-profile retro-fit. The workshop offered to paint them any colour, and I briefly considered matching the red of the tail lights and reflectors. But I chickened-out and went for gloss black.