Anytime the Main/Color Mode, setting is grayed out and the popup is set to Color you are getting a profile conversion with ACPU in the driver/os. Depending on what that profile is you may or may not notice it, so if you want to test this, use different media settings (especially ones that are listed in the ColorSync Utility the have a corresponding profile) and see if it changes the printed target noticeably.
First, I’ll certainly try fixing this setting for APCU but I’m confused about the statement
Depending on what that profile is you may or may not notice it. The target is untagged and I have no profile selection for ACPU so I’m confused. I’m also somewhat confused by the current behavior as CS5 and ACPU produce some of the best measurement correlation of all my tests so far:
CS5 vs. ACPU:
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Average dE: 0.16
Max dE: 0.79
Min dE: 0.01
StdDev dE: 0.10
Best 90% - (836 colors)
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Average dE: 0.14
Max dE: 0.29
Min dE: 0.01
StdDev dE: 0.07
Worst 10% - (94 colors)
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Average dE: 0.37
Max dE: 0.79
Min dE: 0.29
StdDev dE: 0.09
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Very similar to the results on the Epson with good correlation of the two apps, low max dE. For reference, on the 3880, the dE was: Average dE: 0.19 Max dE: 0.72