There might be some additional latency when using L1 due to the need to translate the global address you are accessing to a cache location, however I haven’t run any microbenchmarks to check that.
As to why the 64 kB can’t be assigned to all shared memory or all L1, I’m also not sure. I was genuinely surprised that NVIDIA gave us the option to configure the L1/shared memory ratio at all, given that every feature adds complexity and engineering time.