I thought the idea of per-user /tmp was that every user got his own /tmp, sure, but this was implemented via subdirectories of the *real*, tmpfs, cleared-on-boot /tmp, e.g. /tmp/user-$name/... This can all be done fairly easily with pam_namespace: there's even an example in the default /etc/security/namespace.conf.
(One application that becomes a lot faster with /tmp on tmpfs is GCC without -pipe, or, even with -pipe, at the LTO link step. It writes really quite a lot of large extremely temporary intermediate output to files in /tmp in each stage of the processing pipeline, then reads it back again in the next stage.)