We configure our own kernel and I've been down the path of configuring
x32 there, but afterwards, getting the user land to play nicely is
another matter.

Mind you, I was doing this on EL6, and so I needed to introduce a new
glibc that co-existed harmoniously with the upstream one, and then
bootstrap and build GCC with x32 support. Because we were evaluating
software that already existed for x86_64, I patched Yum and RPM to
accept x32 as an additional architecture.

After all that, I finally ran evaluations on the software we planned
to evaluate in x32, and found the performance gains were negligible
(it was twemcache, which uses tons of pointers, fyi). I highly
recommend doing your own evaluation first, before requesting such a
large change to be applied community-wide. It may not be the panacea
you seek.