After discussing with my team, the general feeling is that keeping protobuf would be a preferred choice. The reason, as i stated earlier, is that proto provides a better standardized description of the API than JSON and also we could create data model rather quickly for bindings/tools written in other languages. We have already generated the go-spdk based upon the proto model for OpenSDS's interaction with SPDK drivers. It provides benefits at least judging from our own practice.
Frankly given gRPC's wide adoption I don't think this should be a big issue. If you still have doubt about this, I think maybe we could setup a conf call, or I could discuss with Harris when he's in China for SPDK summit.
For repo, I think it is entirely up to the community's decision on whether maintain it in the main repo or creating a new one.