Thanks for the detailed numbers!
It looks like the table mirroring achieves very good performance. Firefox without asm validation and with the table mirrors reaches the highest value, 75076000.
As to why Firefox with asm validation is slower, it's likely that this is a microbenchmark that stresses JS calling into the asm.js module. That has higher overhead when asm.js opts are on. In a large app where each such call does enough work it should be worth it, but on a test like this, this is the expected result.
Less clear is why Chrome does so poorly - its best result is 27566000, almost 3x slower than Firefox. Might be worth filing a bug for them to take a look.
It's slightly curious that fastcomp (with mirrored tables) is a little slower on Chrome than oldcomp. However given the almost 3x slowness in the last paragraph, it could be noise compared to whatever bigger issue exists there.
Anyhow, overall, it looks like moving embind to mirrored function tables on fastcomp is a good thing, aside from the code size change. With that, fastcomp is almost as good as oldcomp on Chrome, and better on Firefox.
That leaves the issue of exposing function tables in asm.js itself as mostly a code size issue, not a perf issue. I agree that would be the optimal result, however, speccing exposing the function tables is not trivial (they shouldn't be modifiable from outside, for example).
How big was the code size change with mirrored tables?