I though I would post my experience here in case anyone else runs into a similar problem:
I had a custom profile made for Epson Premium Luster on my Canon ipf6300. The profiling target was printed through the plugin. When I got the profile back and installed it I made a print of an evaluation image using Lightroom and the printer driver, since I knew that the profile could be used with either the 16 bit plugin or 8 bit printer driver. While the colors looked very good, the gray axis was clearly warm to my eye, particularly in the region of L* 30-60. When measured with my Eye One Pro, a mid neutral gray had a b* value of around 3, whereas it should have been around -1 to -1.5 as the b* value of this particular paper has a white point with b* about -3.
I tried various options to figure out what was wrong. Finally, I decided to check the evaluation image printed through the plugin. The grays were obviously neutral. I went back to check the driver settings and found that I had checked "High Precision Photographs" in the driver--a setting that doesn't exist in the plugin. Once I unchecked this setting, the prints from the plugin and the driver (using Lightroom) matched and were both neutral.
Moral of the story: Don't check "High Precision Photographs" when printing through the driver!
PS I hope this will save someone else the time and hassle I went through.
