I'm leaning towards the theory that there are just a few defective z3100's out there as I've only seen a very few individuals having problems with paper marks, and some of those individuals were using wrong paper settings (some of those were solved here on the forums).
I've printed on about 8 papers (and canvas and vinyl), thick and thin, and have yet to see the first paper mark even though I'm in humid Georgia. I have seen marks on profiling charts, but I believe those marks are created when the printer pulls the profile back into the printer to read the color samples. I do keep the printer and my papers in an isolated room (formerly my darkroom!) and run the AC when necessary. My house is very well insulated, has exposed concrete floors and very high ceilings, so it doesn't get that hot even with no AC and my "printing room" even more so as it has no windows. Climate control of the printing environment isn't "very expensive and not ecological," it is a near necessity if one is aiming for consistent printing for business. This was true for film and chemistry and darkroom printing and it is just as important for digital printing. Paper will absorb humidity and it simply isn't going to perform the same when it is stored properly in plastic in a controlled environment as when it is left exposed to heat and high humidity. The same was true of traditional photographic papers and films. Extreme heat and humidity aren't just bad for papers, it's bad for your printers, computers, monitors, etc. Literally all computers, components, printers and electronics are shipped with silica gel precisely because humidity is the enemy of all the gizmos we expect to work all the time and, in shipping, manufacturers know these items are likely to endure some warehouse / shipping conditions that are less than ideal, i.e. hot and humid.
That said, I lean towards the "few defective z's" theory, because of the thousands of z's sold, I've seen maybe 10-12 individuals here and elsewhere with transport marks that don't appear to be the result of user error.
I haven't printed on any of the new baryta papers, but I'm unlikely to because I love my B&W's on HP's Photo ID Satin. I'm doing editions so consistency is critical and changing papers is a no-no. I can't imagine paying at least twice as much for the unproven baryta papers (I'm unaware of any archival testing data for any of these papers) when the HP ID Satin gives me the look of a traditional fiber B&W...not the feel, certainly, but the look is perfect. I have samples of the same image from the same 4x5 negs. printed on traditional Ilford Fiber Multi-Grade Glossy and HP Photo ID Satin and on the wall they are virtually indistinguishable as to which is digital and which is darkroom. I like that the HP Satin lies nice and flat also, much easier for framing, especially at the sizes I am printing - 32" x 40", 40" x 50".
I think a company could make a nice living just by offering a new paper of some sort every 3-6 months as there seems to be a market for the "new." It was inconsistency that drove me from Kodak films and B&W papers some years ago - every year something was "new" or "improved" with new processing times, look, packaging, etc. This made consistent results difficult if not impossible with Kodak products. Kodak's "strategy" practically gave the title of "reliable and consistent brand" to Ilford.
All that said, I hope HP can definitively solve the problems for those that have them. Some of you are getting paper marks and there has to be a reason for that. And it's quite possible my completely satisfactorily results with the z3100 are related to my more "consistency centered" approach in my workflow. I'm not trying a lot of papers, certainly not when they first hit the market. I stick to a stable of about 6-8, and for my own work, the vast majority of which is B&W, I'm set on HP Photo ID Satin because the look is outstanding, I've already committed quite a few editions of oversized prints to it, and I want consistency in appearance when I show these images, now and in the future.