Will Gentoo users require a massive recompile due to the ABI change?
Probably. See here [0] for more info. And Gentoo has some nice tools to help figure out what needs to be re-compiled.
Usually, my laptop can recompile @world (aka everything) overnight, so it's not too much of an issue.
[0] http://packages.gentoo.org/package/sys-devel/gcc