Thought about it when filing bug 930213. We could probably compress binpkgs asynchronously. Right now, we don't consider the result of the merge usable until the binpkg compression is done, but there's no real need for that, we could just compress in the background and continue as the package is already merged well enough for other packages to use it as a dependency.
I should say, this only really matters with emerge --jobs I guess, or if it's not the last package in the merge order. If it's the last package, it of course won't help.
You'd basically need a backup copy of the whole PORTAGE_BUILDDIR though (not as bad if filesystem supports reflinks) since the merge process calls movefile which does not leave the source file behind, and also the pkg_preinst phase can muck around in ${D} (plus ${T}/environment caries state from pkg_preinst to the installed instance).
Thanks, that makes sense. I agree it's not worth it.