I've not used an HP printer for a few years but I know a few people who run Z2100 & Z3200 printers using both the standard Windows drivers & also EFI ColorProof XF RIP & also one person who used to use ImagePrint RIP but now uses the Windows driver.
In terms of quality of prints, it really depends if you are selling the prints in an exhibition/gallery setting & have such demanding clients that they will actually notice any difference at all (there are so few buyers that fall into this group it is not worth even concerning yourself about them) that matters. I could just about guarantee that anyone inspecting your prints that closely will definitely not be paying good money to purchase said prints.
Any real, actual paying customer who wants the print to hang on their wall or put into a shop or somehwere similar could not care in the slightest what the print quality will be like because these printers produce such high quality output that it would be all but invisible to anyone except people like us. A real paying customer wants a good looking print on their wall, not a "high quality" print - if the image is a great image, the print (as long as you have some idea what you're doing) will look great.
Getting back to your initial concern, there used to be subtle improvements possible with ImagePrint when printing B&W but now the drivers & profiling apps are sufficiently advanced that the improvements, if there are any, are very difficult to see without direct side-by-side comparison. Show someone a print made with the standard driver & the same image printed through EFI XF or ImagePrint & there is no way they could tell which was which.
I am currently in the process of linearizing & profiling a few papers used with my 3880 & EFI ColorProof XF 4.5 to see for myself if there are any tangible image quality improvements over the Windows driver & careful post-processing/photoshop printing. It's going to take me a few weeks but I really don't know if there will be much, if any improvement. Now, XF 4.5, when using the RIP/printer as RGB actually does in fact use the manufacturers own ink mixing/screening/ink limits/etc & not the RIPs own specific recipe. It is only when using the printer/RIP as halftone CMYK that the user even has anything they can alter concerning ink limits, etc. That is how far advanced the manufacturers settings & drivers are these days.
So really, best thing & in the end, the only thing you can do is to make the comparisons for yourself on your own printer. No matter what I or anyone else says, it still comes down to your abilities & the fact that the Z2100 & standard driver is so good anyway. As long as you are carefully printing & measuring your profile targets & choosing the "correct" settings in the profiling app & driver settings, your output should be up to almost anyones standards. I'm not referring to proofing & delta E, I am talking about actual visible image quality. There just isn't much in it these days.
Let me know how you go & if you need a hand or have some questions, ask away. Good luck & persevere & hopefully you will come to similar conclusions. What RIP are you thinking of using?
P.S. forgot to mention that every profiling app & RIP (or just about all) produced in the past 2-3 years or so will use the built-in spectro.