I have a 7800 set up with a Piezo2 WN-SEL inkset. Most gloss prints are done with Hahnemuhle Photo Rag Pearl, and occasionally with Fine Art Pearl. I am using the Shade 1 Opaque Photo Black. IJM made a custom curve a couple years ago for our 9800 that uses the Selenium Shade 1 Photo Black, and I've been using that same curve on the 7800 with Opaque Photo Black installed. I normally dry with a hairdryer for 3-5 minutes before applying GO with the GO3000 curve. Sometimes if its a big print I'll let it sit overnight after the hair dryer step.
I've noticed that printing on the 7800 with PR Pearl and FA Pearl that I get gloss differential in the darkest tones of the image (the ones at 0,0,0 that are actually printing with the Shade 1 ink). I've tried it with other GO curves (up to 40000 units). I even applied a double coat of GO to one print and it didn't fully cover the blacks...However if I adjust the darkest tones so they print more like 8,8,8, I don't have the gloss differential coming though.
Since the curve was made for a different (selenium) Shade 1 ink, and this printer has the Opaque Photo Black in Shade 1, is it possible that the curve is putting down too much shade 1 ink and causing this? Maybe a custom curve would fix this?