Talking to a friend this week who told me that he always uses colour mode for his black and white prints on the 3800. He felt that metamerism was no longer a problem. Mind you he's mostly a colour photographer.
I decided to check for myself. Here's my observations.
In 16 bit Photoshop I made an image 2X8 inche, 300 dpi and used a white to light gray gradient. I then printed it two ways, first in colour, and then I printed it again, in advanced black and white mode. I used the appropriate profile for the Harman gloss paper in colour, and no profile in black and white.
The gradients didn't match in darkness which didn't surprise me but what I was looking for was overall colour to this completely gray print and also to look under magnification at how much colour was included in the printing of the two gradients.
1) both prints looked neutral gray to my eye
2) the advanced black and white printing was slightly warmer than the other - I'd say neutral was somewhere inbetween and we are talking subtleties here. You would not notice exc. for side by side comparisons.
3) under magnification, both used quite a lot of blue, perhaps a bit more in the colour print. The colour print included a little magenta and perhaps a tiny amount of yellow (I wasn't really sure - under 8X magnification I couldn't be sure).
4) I then took my prints around the house on a sunny day, checking out sun, north shade with blue sky, incandescent and fluorescent. In comparison with prints made on the 4000 (my old standard printer) colour prints showed minimal metamerism - a really tiny amount which would not be an issue in my opinion.
Now, Paul Roark makes a big thing of how much colour ink is used in advanced black and white mode, making the point that the dedicated inks for black and white printing are so much better, but here's the thing.
Carbon isn't black - it's brown - ie. warmer than neutral. The only way to make neutral prints with carbon is to add something bluish to cancel out the warmth.
Sure you can mix the ink in the bottle and the dots printed will be neutral gray - but they still contain blue ink. With colour printers, the gray dots are mixed in with the blue dots (and a smidge of magenta) to get neutral so each dot is it's own colour, not a mix (that's the nature of combining dots instead of blending inks). Since these colour dots are not visible to the naked eye, the end result is the same.
So: unless you plan to use pure carbon inks and live with very warm tone prints, you are going to be using blue/cyan somewhere along the process and frankly I'm not convinced that in the end it makes any difference whether you use dedicated inks or not.
The next enquiry I need to make is to further assess just how much more colour ink is in the colour print vs. the abw print because this has longeivity issues - carbon being more stable than colour inks (whether blended in the bottle or not).
The last concern is whether the abw mode actually handles highlights and shadows any better than colour mode - that's a test for another day.