I'm involved in a project of some extreme colors for reproduction using the machines mentioned. My searches found this thread. I made these photographs this morning after breakfast. Some of the profiles I find online concern me.
Yeah. They depend on the stability of the machine process and how much change has occurred since the profile was made. Quite apart from some profiles looking like they may not be well formed.
I just refined my profile target for the 9800. I make a profile then cross check it with independent colors printed in 16 bit tiffs using ProPhoto. The approach most use is to just run Abs. Col. on the target RGB values and compare against the scanned LAB values. That greatly overstates accuracy because it depends only on the A2B1 tables which are the most precise. It's a tough nut but I've finally come up with a target that produces excellent results in both the neutrals and the rest of the gamut. Neutrals come in at ave dE00 of .33 and colors: ave dE .27. dE00 is more sensitive than dE76 on the neutrals but much less sensitive as the patches' chroma increases. The 9800 has a couple sharp gradient changes in L* and abrupt shifts on a* and b* as well. That extra page of near neutrals helped quite a bit.
It's been quite hard to get the neutrals below .5
I wound up with a target made out of the original, default single page of 957 iSiS patches. Then added 1914 (two more pages) made out of I1Profiler's optimization process. Then I added another 957 patch page of neutrals and near neutrals with 3x redundancy on the near neutrals. The near neutrals had RGB intervals that matched the 37 point I1P grid which I speculate should provide somewhat better accuracy in the A2B1 tables which have evenly spaced RGB points.
This provides a good, 4 page, target set for my 9800. At least on glossy type media.
The general rule of thumb is an average dE76 of 2.5 in a complex image is roughly at the "perceptible" point. It's a lot lower for certain images though and I'm mostly curious just to find out what is possible.
Oh, and the other thing I do is scan the pages both forward and backwards. I make targets with reference bars on the bottom as well as the top to enable this with the iSiS. Averaging these provides significant improvements over just re-reading them in the same direction. Quite a bit more than I anticipated. I have some theories as to why based on some other tests to determine the effective scan area within a patch. That's another topic though.