Ingvar Stepanyan, author of the original 6to5 pull request has finished submitting a series of further optimizations via https://github.com/6to5/6to5/pull/736.
I'm clocking a 110-215X performance boost: 1,255,700 - 2,384,556 ops/sec! (on Chrome and Firefox alpha builds respectively) Check out the new JSPerf: http://jsperf.com/tco/17
That's quite a speed-up, isn't it? It looks like the newly optimised version of 6to5 effectively inlines the tail call, similar to the way a real compiler would, and so avoids function call overheads altogether. Smart move. I hadn't realised the processing done by transpilers like 6to5 was so sophisticated, and I have even more respect for their developers now than I already did.
