It's been a while since I've played with Gentoo, I simply don't have time to watch my whole system compile.
If you decide to go the Debian route, there is still plenty of source code to play with. While Debian isn't meant to be installed like a Gentoo system, every binary in the official repository should have an accompanying source package. apt-get is normally used to get and install binary packages, but it can also be used to
compile and install source packages from the repository. Also there are other tools, such as apt-src, and apt-build. You could use apt-build world to recompile the entire system for optimization if desired.
Edit: Debian also has tools for dealing with source not in the repository, such as debhelper.
And while Arch Linux was mentioned, I have little experience with it, but Arch as their
AUR which deals with source code.