Jim,
Having run a number of prints and comparing StudioPrint output to the HP driver output (without APS, which I don't own) I am seeing:
1. On b/w prints, more accurate shadow detail when using a quad K print envronment in StudioPrint
2. On general color images, like portraits, PDI test images, slightly cleaner and more accurate colors with StudioPrint
3. On difficult color gradations, ie saturated colors transitioning to black, Granger rainbows, etc the nod goes to the HP driver. It is not more accurate, but more pleasing, as StudioPrint shows some posterizing in the transitions.
Currently, StudioPrint does not run through the built-in spectro. My understanding is that it is in the plans, but can't say how far out that is. If I build a quad K environment in StudioPrint I can hit very subtle shadow separation that get blocked by the HP driver. We're talking very subtle shadow detail, and to be honest, it took a number of images before I hit this limitation with the HP driver running the 4.1.02 firmware. Overall, with the papers optimzed in the newest firmware, I think the HP driver and profiler does a very good job for an RGB profiling solution. At least now it compares much better against PrintFix Pro, which is surprisingly good, considering the price point.
There are benefits to driving a CMYK printer as a CMYK device, but there is a lot of work involved too. You've got to know how to linearize, set ink limits, and then have the equipment to measure fairly large patch counts for color profiles. Is it worth it? On some images, you say yes, definitely. On others, the differences are so close, you say why bother? And in a few instances I am getting more pleasing results from the HP driver. My sense is that all of the RIP companies have their work cut out for them with the CMYKRGB printers. A much different beast that the standard 4c inksets. I'm still working through some issues with StudioPrint profiling behavior. Anyone who's complaining about feeling like a guinea pig with the HP driver/profiling would not want to all of a sudden jump into a RIP right now. RIP support is still limited to very few vendors. A few weeks or a couple of months from now, it will likely be a different story.
I think that for a large majority of images the HP driver/profiler does a commendable job. StudioPrint offers some advantages today, but is not the magic bullet for all types of images yet. I'm still reserving judgement as to the route I will ultimately take with driving the Z. Unless you are completely unhappy with the HP OEM solution I would give it a bit of time to settle out.