Well, after getting tired trying to get more out of an I1Pro 2 than it perhaps can offer, I bought the XL. and am putting it though its paces to understand its capabilities as well as limits. Initial tests show a few things:
1. The instrument produces highly repeatable chart readings. The Isis default patch set, 957 patches, fits on letter size sheet. Successive reads produced average dEs of 0.04, and a max dE of .45 on glossy paper. This compares to about .20 with a max of 1.9 with the I1Pro 2 on a 918 patch set with two sheets. Much of this is likely due to the precision feeding and automatic registration of the Isis. So it is reading almost exactly the same locations on the patches. Reading patches printed on different sheets shifting the image slightly produces variations similar to the I1Pro.
2. I hand measured a set of 64 RGB patches evenly spread through the RGB gamut with a distance of 85 (see patchtool) on an Isis target and compared them to the Isis measurements. The average dE was .45 and max was .9. This was better than I expected as my I1Pro 2 is over 5 years old and has never been factory calibrated. Remarkable, really.
Now for the bad news.
The Isis moves paper using bottom wheels spaced about 2 cm apart and top wheels that are spaced 4.2 cm apart and the top ones can leave "tracks." [added: The 4.2 cm "top wheels" are fixed and seem to be for the purpose of keeping the paper flat] For glossy prints that have dried for 24 hours, the tracks are nearly invisible and there is no significant impact on the Isis readings. But for thick Luster papers the wheels appear to flatten the paper and create a region, about 1mm wide, where the colors appear, and measure, a more saturated color. For instance a light yellow patch (RGB 255,255,170) produced a b* of 32.40 but on a patch with a tread running through it the b* was 35.54. Wow!
However, that paper, a Baryta paper, is 300gsm and 16mils thick (.41 mm) which is just under the max thickness of .45 mm the XL can handle. It is likely a smaller effect with thinner paper and is not a significant effect with glossy.
The good news is that the treads are spaced such that it only affects one of every 7 columns so one possible solution is to create a specific patch set that fills the columns were the tread wheels track with blank patches. This would decrease the max number of useable patches by 1/7th but would be pretty solid. This is pretty easy to do with a program script to manipulate CGAT files.
A second approach would be to shift the paper to either the left or right such that the wheel marks were centered on the transition where one patch goes to the next. This should work quite well if the paper alignment is sufficiently repeatable. It has to pretty much ignore readings within 1.3mm or so of that transition to work. I'm currently testing that.
Here's a cell snap showing the wheel marks on Finestra BaryTa Fine Art 16mil paper. This was taken in sunlight arranged so that the reflectance showed the wheel marks. There are extra tracks a cm or so apart in the image as I read the paper offset about 1 cm to see the effect of additional tracks.
As an aside, the effect is most noticeable on more saturated colors and not the neutral grays. The dE 2000 differences are much smaller. < 50%. So I doubt this would be visible in photo prints. Overall, it's still better, on average, than manual scanning with an I1 Pro2 and far better with glossy.