I think this is very useful information. Thanks for your help. The features between the P400 and P600 seem very, very close. However, the P600 offers 1 more ink cartridge, but costs more to maintain (ink cartridges + piezoflush fluid vs. fluid alone for the P400) and by heft, is a studier machine. Don't know if there's a difference in the reliability of paper handling between the two or not, but I've worried the P400 might be a bit lightweight for that heavier matte papers I tend to like... but that may be unfair.
Stacked up side by side, the difference to 1st Piezo use looks to run (with current rebates from Epson) about $ 300 or $400 depending on whether or not you dupe the P600 methodology for cleaning the P400. This assumes you already have QuadtoneRIP and don't do the Curve Software - these throw in another $200 together. Add a box of 50 sheets of Cone paper at $70, and if I'm at all close, I come up with roughly $1,150 for the P400 and $1,550 for the P600 for all-in purchase to first print cost. Does this sound close?
If so, then the question is whether one extra cartridge of ink is worth the difference? Fairly, if you were to maintain the P400 the same way with piezoflush loaded cartridges 'cause it'd be a better idea, this adds more to the P400 cost and narrows the difference.While I'm not sure this would make sense or actually add to the reliability, if it was a "better" idea, then the P600 becomes a clear winner.
For me, the consideration of moving down to a smaller printer for 3rd party inks - specifically Piezography means that 13 X 19 or whatever length has to be able to prove itself as sufficient as a "proof" that could mean you've got it 95% or better "right" for printing your could print larger (if needed) for a final image with a 3rd party. My current use is mostly B&W coming off a square negative scanned on a Nikon LS8000... which dates my approach, yes, but suits my current needs.
Thanks again. I'm glad your system is out there, and look to go down this road... taking steps to get started before the end of the month.