The centralized approach they're using does this without exploding in size. I think what you want would be thread local tables to avoid locks/syncing across threads? Would only help for mt, though.

Writing to a ring buffer and processing in batches should be a relatively big W for everything. You could then filter for repeated instructions to compress, which would lend itself to prefetching the next instruction and buffer with hints. Mixed precision record storage might help too, but you're plucking hairs at that point. Making the sampling adaptive based on the hw profiling counters might eliminate some 'useless' work.

The overhead is already so low that it really should not matter, though. Used to spend a lot of time trying to find wins like this but caches have gotten so large it basically doesn't matter. Even in pathologically memory or io bandwidth bound cases I've found it's usually faster to just run 2 or 4 smaller instances over trying to coordinate all of the threads in a single big one.