Have you tried your matt paper with the photo black ink? For many, the prints are still very nice with many papers. Worth a test before changing the ink over.
In doing my own testing with my 3880, 9800 & 9900, though the MK showed a slightly richer black, the same image on the same paper with PK was still quite nice and was a very acceptable print.
Showing my samples to many folks coming to my studio, few could discern the difference and those who could see the difference didn't think it mattered too much.