I've been using Slackware 10.1 and slackpkg to sync with -current to keep it more or less up to date. However, I am now considering upgrading glibc.

1) What are the benefits of upgrading glibc?
2) Can I use the standard command <i>slackpkg upgrade glibc</i> or do I have to manually download and install it?
3) Will it break my official Slackware binaries?
4) Will it break my self-compiled binaries?
5) Which glibc packages must I install?

Lastly, will there be any large-scale performance advantage (if any) if I compile it from source (something like the magnitude of compiling the kernel from source)?

I don't have any answer, but you should read that:
ftp://ftp.slackware.at/slackware-10.2/UPGRADE.TXT

Did you ever do this upgrade? I tried upgrading from source and the compile worked, the testsuite ran fine, but the install failed.

Peace...

you will get bugfixes from the previous version and (i hope) more stable library that will be used all around your system. Besides that, glibc in Slackware is connected with other packages as well, such as GCC and some other library, so if you update GCC, for example, it's best to upgrade the glibc also

i usually download it manually and use upgradepkg *.tgz to upgrade all packages

well... i'm not sure about it, but in case of my system, it won't break anything. Just upgrade, run ldconfig, and everything is back to normal again

perhaps, so you should have a backup of your previous (current) glibc, just in case newer version breaks some binaries

the latest stable in Slackware's repository is 2.3.6 is think