I just printed 28 large pieces (up to 20x45) for a show and I can't believe how little ink I used.
Does the HP give you a status report with the actual ml used?
I agree 100% on the inks.
Every non-OEM manufacturer makes these wholly unsubstatiated bs claims about gamut and longevity. It is really time that they step up to the plate and publish some test results!
Plus provide a decent set of ICC profiles so that you don't have to create all of your own profiules (the claim is often that thses are a direct switch, no new profiles required. Wrong!)
Even I know how to do the testing in my basement. It doesn't cost more than $100, and the procedures are pretty well documented by now. They would all rather market based on pure bs than do the work to establish the truth.
There is really no reason to accept that nowadays with 100,000 other ink providers out there ....