hello jondeleon,
according with manual "Shared memory default 2GB. Max Shared memory supports up to 16GB.* The Max shared memory 16GB requires 32GB system memory installed." so we can expect to go higher. You should contact Asrock support if you don't find more than 2GB on BIOS Vram selection.
But increasing this VRAM not result in performance boost because Windows already use the same memory speed and switch VRAM content with standard RAM when needed. techspot have published many tests with VRAM, you can take a look.
techspot.com/article/1578-amd-raven-ridge-reserved-memory-explainer/
