There's nothing _significantly useful_ to learn from Gentoo that you cannot
learn from Arch. In fact, they serve the same crowd, except the latter
serves the less patient. The competency required to set up _and_ maintain is
similar for both. You are required to do a lot of reading, and some thinking
in case you find yourself in a pinch. Neither is what I would call
"hardcore", because hardcore in terms of computing is something like
http://www.coreboot.org/ or designing your own kernel. Even a project like
LFS is not "hardcore", it's "educational".
With that said, there's no reason you couldn't have configured a Gentoo
system if you managed to configure Arch (aside from time and patience of
course). However, why someone at this point of time would be at such a
position is due to the Gentoo Wiki's current state. As you may or may not
know, there was a problem with the hosting/server and most if not all of the
content had to be ported to a new database. I used to thank the Gentoo
community for the _largest_ documentation one could ever find, but now it's
just a mess. Sure, you could get some of them from
http://www.gentoo-wiki.info/, but it's no longer the same. There were plenty
of helpful material that newbies to Gentoo couldn't do without.
Anyone who has been running Gentoo can continue to do so, but if you're
planning to deploy a platform anew I'd advise against Gentoo. If the trouble
is reading documentation or competency, then Sabayon is a better bet than
even Arch. You still have the same control over your system which is
necessary for most of the Linux audio crowd.
Now about hybrid architectures, _a lot_ of distros allow for multilib setups
even if they come native by default. On the other hand, Arch does not, and
will not for the foreseeable future, support multilib. That is purely
technical, as running a hybrid system is actually tainting it [[
http://wiki.archlinux.org/index.php/Current_state_of_Arch64_developement ]].
But the idea of a 64-bit kernel with a 32-bit userland is interesting.