From Colorbyte's web site regarding Phatte Black:

"With the Phatte Black option, light-light black isn't used, but it won't be missed--print's made with Phatte Black show virtually no difference when compared with prints made with the standard configuration."

'virtually no difference' - a fairly non-committal/un-inspiring statement no? virtually no difference sounds like there are (negative) differences, perhaps not significant though. It will be interesting to hear from people running Imageprint and comparing their results over a range of paper types using Phatte Black vs the full K3 inkset.