It's not just gamut volume, it's also how colours within that volume are rendered. Somewhat obviously, an orange ink is useful in things like sunsets, because you have orange ink to use - call it a dark yellow in a way, it means you don't need to use yellow-and-something-else to create an orange - you start with orange. This is most clear in smooth gradations of such colour ranges.
Whether it affects any particular image you'd have to test, and whether it affects a signficant number of images from a given photog, again you'd have to test. But there's certainly more to it than just numerical gamut volumes and in particular more to it than just gamut volume boundaries.
Hi Phil, I'm standing here comparing the outputs of the Atkinson test page for Epson 3800 vs 4900 under D50 Solux illumination - the sunset image, the orangy rock structure image, the fall tree leaves image, and the smooth colour ramps (orange not being one of them, but red and yellow are represented) and the difference is ZILCH. And my visual perception of fine colour differences is very acute, even at my age! And to boot the 7890 has Vivid Magenta, which the 3800 didn't have. Assuming the 7890 is even better than my 3800 was, I just can't see what Chris has to gain of practical significance spending an extra thousand Euros on a 7890 versus a 7900. The image quality of all these machines is so close that in the final analysis in this kind of situation it really comes down to price - unless you would suggest any major differences in build quality having a real impact on prospective longevity.