Applying GO for Piezo is simple. Just print a uniformly blank square with the GO curve, GO will cover all the printable area of the paper. I agree it does work very well on some papers, but try on Hahnemuhle Photo Rag Satin, Moab Colorado Satin, Moab Colorado Gloss and the deep blacks patches remain stubbornly matte.
For the shadows, I understand what you mean. I have done digital B&W printing since 2003, starting with Piezography on an Epson 1280. Well, by setting the shadow tonality and maximum print desity appropriately in the ABW driver, followed by profiling, the shadow detail and contrast is just as good with ABW compared to Piezo.
Piezo is generally noticeably better than ABW with default settings, but not compared to ABW properly tuned and profiled.
I should ask that being on Windows instead of Mac seems to have an importance. I never experienced printing artefacts when using Piezo or MIS inks with QTR on a Mac, but on Windows, you have to rely on QTRGui which appears to be on light maintenance at best and is not reliable.
I also did not try to get custom curves from Cone.