Not much room for such sharing to exist, every Skylake core can do at least 32 FLOPs per cycle simultaneously.

Besides, it'd be obvious in multi-threaded benchmarks.

Shared units between cores can be done and has been done, by AMD. It's hard; AMD didn't do a great job of it. Intel didn't even try.

And AMD has given up on it, their next-gen Zen CPU will use a more hyperthreading-alike architecture.

Only caches and memory/IO controllers are usually shared between cores.