2) Have you considered downloading the all-of-Hackage tarball and

running haskell-src-exts over it to get a benchmark of how much HSE can
already parse of the Haskell code out there?

Considered, yes. Done, no. Would love to see the results :-). The crew at OdHac (Roman, Erik, Simon) ensured that the current version handles all of 'base', which is a good start.