I think that it'd be a lot easier to justify increased line speeds if the level crossing situation was sorted out. There may be some electric 125mph stock available in a few years time (i.e. the 91/ Mk4s, depending on the IEP situation), which would give the GEML a decent case for an upgrade.
Even 90s can do 110 okay, so there's scope for speed improvements with the existing stock.
But one problem it'll always have is that the fast Norwich service is going to be the only "Inter City" service for the Liverpool Street TOC (however you split up TOC boundaries), which may make EMUs more tempting.
(as for "topography" - I know a few people who moved from East Anglia to South Yorkshire when Norwich Union were shifting jobs around a few years ago and like to complain about the hills here

)