Yeah, so, I did my own test about the colour mixing. And I'm pretty upset at HP at this point. Here's my methodology:
Tested Paper: Staples Supreme Photo Satin
Profiling tool: X-Rite DTP20 (handheld) with Pulse Color Elite RGB
Config 1 (Generic): Custom paper type as "Photo Paper -> Photo Semi-Gloss/Satin Paper"
Config 2 (HP): Lied to the driver; said it was "Photo Paper -> HP Pro Satin Photo"
For each configuration, I ran a calibration/linearization, then printed the 729 patch calibration target from Monaco Pulse ColorElite. Print driver set to "application managed colour".
The linearization targets looked essentially identical, as I expected. But even to my eyes, the calibration targets were *vastly* different. The HP config had much deeper and richer colours and much better contrast. I don't usually expect to be able to so easily differentiate profiling targets, but the difference is amazing.
After creating the ICM profiles for each configuration, I loaded them up in GamutWorks for review.
The Generic config has a volume of 1,317K while the HP config has a volume of 1,531K. Most of the additional volume is in the deep saturated colours, but surprisingly there is also substantial more gamut in the lighter colours. It is like a whole new printer.
I have a brutally broad gamut floral print with extremely saturated magentas and purples (far outside the gamut of my displays, in fact, thanks to ProPhoto) that was totally destroyed by the generic config. Printed again with the HP config, it approaches the prints from my dye-based Canon equipment (which is nothing short of remarkable).
So, why am I upset at HP? Locking the better capabilities of the printer/driver to HP branded papers, when those capabilities are clearly applicable to all papers, is deceptive. A naive user will be discouraged from using third party papers if they go through the recommended "add custom paper" setup routine, because the output is poor. When, technologically speaking, there is no need for poor quality output on custom papers at all. The printer is capable of doing better -- you just have to tell it you're using HP-branded paper, and "voila!" The output is better!
Continuing on in this approach -- lying to the printer and the driver about the paper type because the generic-based custom paper types result in inferior prints -- is not acceptable. I would need to re-linearize my chosen HP paper type each time I used a new third-party paper in that slot (although I could keep my ICC profiles), because there's no way to duplicate one of those slots.
I expect HP to open up the full capability of this printer to the generic paper types ASAP, or I will not be a very happy customer.
N.B. I *do* like the prints. But the technical tomfoolery required to get them is reproachful.
I would be happy to mail the two ICM profiles (Generic and HP) to anyone interested.