Since binary packages are [strongly] compressed anyway, I don't think there's a point in having docs compressed before creating the package. In fact, keeping them uncompressed gives us two advantages: 1. improves binpkg compression ratio and speed, 2. makes binpkgs respect PORTAGE_COMPRESS of target host. Therefore, I think it'd be beneficial to move compression after binary package is created by FEATURES=buildpkg, and enable it for binpkg install. Backwards compatibility: our fancy functions detect already-compressed files, so handling old binpkgs should not be a problem. New binpackages on old systems will just lose doc compression which is a minor issue. One thing I'm wondering about are prep* calls that some ebuilds are still doing.
