> which (aside from making VM development more complicated) significantly increases lock traffic and thus runtime costs, which noticeably impacts single-threaded performance, which is of major import.

I don't think that's a fair characterization of the trade offs. Acquiring uncontended mutexes is basically free (and fairly side-effect free) so single-threaded performance will not be noticeably impacted.

Every large C project I'm aware of (read: kernels) that has publicly switched from coarse locks to fine-grained locks has considered it to be a huge win with little to no impact on single-threaded performance. You can even gain performance if you chop up objects or allocations into finer-grained blobs to fit your finer-grained locking strategy because it can play nicer with cache friendliness (accessing one bit of code doesn't kick the other bits of code out of the cache).