I've used Cap'n'Proto for a work project, and aside from not-always-amazing language compatibility it really is awesome. It's what I wish Protobufs was. The performance improvements don't matter much to me, the biggest thing for me is the type system fixes most of the mistakes that are mentioned in this article.

is it possible to use this with gRPC?

edit: seems, not possible as of now - https://github.com/capnproto/capnproto/issues/478