I think you are the most likely person to do this sort of testing.
You can use machines on the GCC compile farm for this.
Your patch to change the symbol table's load factor is fine technically.
I think the argument for putting it in is lacking; what I would like to
see is either some rationale explaining that the increased memory use is
not important, or some numbers showing that it still performs well on
more than a single machine. My reason for wanting this is just that,
historically, GCC has been very sensitive to increases in memory use.
Alternatively, comments from more active maintainers indicating that
they don't care about this would also help your case.
I can't approve or reject the libiberty change, just the libcpp one.