Gentoo on a low-RAM build host: prebuilt binary packages or --buildpkg with local source builds?

I maintain a Gentoo system on a machine with limited RAM and CPU, and large packages like webkit-gtk and libreoffice are impractical to compile from source on every update. Portage documents two relevant paths: pulling prebuilt binary packages via FEATURES="getbinpkg" with a configured binhost, or building from source locally while passing --buildpkg so reusable binaries are produced for future merges.

The trade-off I cannot resolve: the official binhost binaries are built with generic USE flags and a conservative -march, so they may not match my USE configuration, and Portage will silently fall back to source compiles on mismatch. Local --buildpkg keeps my exact USE flags and -march=native, but the first build of each large package still risks OOM failures during linking.

Assume a current Portage version and a stable amd64 profile.

1. For a single low-resource host (no other machines to share binaries with), does --buildpkg offer any real advantage over plain source merges?
2. Can the official binhost be used selectively per-package, so only the oversized packages come from binaries while the rest stay source-built with my USE flags?
3. How do I verify before a world update which packages would actually resolve to a binary rather than a source fallback?