I don't do enough B&W to make a useful judgment.
The things that jumped out most at me on my initial test were tonal smoothness and shadow rendering. The test image, taken in light reflected from a canyon wall at Capitol Gorge in Capitol Reef NP, glowed in a way I'd seen only on my screen until then. And mind you, I saw that even with DEMODEMODEMO printed all over the paper!
The image is here
. This is a web version of the image that doesn't really show the tonal range of the original.
I compared with the Epson 3800/Enhanced Matte using canned Epson profiles, Epson 3800/Hahnemühle Photo Rag 308 using Hahnemühle canned profiles, and Epson 2400/Hahnemühle Photo Rag using a custom Chromix ColorValet profile.
What I saw was much better tonal resolution, which brought a glow to the midtones - the sandstone in the background. The bark of the tree preserved excellent shadow detail. I'd given up on seeing the exposed wood of the tree printed as I visualized, but the IP print pulls it off. Same goes for the transition to darkness in that crevice at the left.
I knew right then I had to have IP.
Since I got a license, I've printed more on Hahnemühle Photo Rag; but also on Epson Enhanced Matte, Epson Premium Luster, Moab Kokopelli Satin, and Pictorico Photo Gallery Film. I couldn't be happier with the quality.
Prints made on luster and gloss paper show less improvement than prints made on matte paper, but all do show improvement.
Of the parameters you list, I would rank the IP improvement in this descending order: tonal smoothness, color accuracy (with relative colorimetric rendering), detail, and dithering. I would add shadow detail near the top as well, probably between tonal smoothness and color accuracy.
My workflow is as follows: Raw converter with no sharpening; output to TIFF; intake into CS2; input sharpening with PhotoKit Sharpener (PKS); any regional edits and "creative sharpening" with PKS; flatten and save print file; resize and interpolate to 360ppi for IP; output sharpening with PKS; flatten again; print with IP.
I've been using the RF3 variant of the IP profiles, intended for general mixed lighting.