Thanks all for the useful replies.
Yes, Czornyj's "be realistic" is a useful sanity check. Also a reminder how far we have to go with our inkjet gamut, before we come close to ARGB. I did wonder about the new 10color (?) small format Epson's announced late last year, but that is not wide enough for a 40" print.
I'll think about the generous HP offers, (thanks Phil!) but it sounds from what's said here, that the gain is marginal, in any case. This does indeed have to be a classic reflection print, and cannot be on metallics or backlit, or whatever.
Yes, it is relative, and with no comparison looks ok, but it really looks amazing on screen, that's the image I want to get closest to, and makes it very hard to accept the trade-down in gamut. I'll do an iPF 8300 print, to compare, but expect to see only the smallest of difference.