Just wanted to let you know that grsecurity-3.1-4.8.6-201611082138.patch contains no definition for SLAB_USERCOPY which causes compile issues with your NVIDIA patch. Was your SLAB_USERCOPY stuff replaced by CONFIG_HARDENED_USERCOPY?
the slab whitelisting mechanism based on SLAB_USERCOPY was replaced with a new API based on kmem_cache_create_usercopy which is what the nvidia shim will have to use from now on.
update: spender will provide backward compatible behaviour in grsec. nvidia should probably stop using their own kernel stacks as upstream moved to 16k ones a while ago.