As I said - marginal. The GotG comparisons don't reflect superior fidelity so much as they do color tweaks, filters, and grading. I actually prefer the older shots there, but all of this is beside the point, as I'm not arguing that it never happens, what I'm saying is that you can't guarantee that it
will always happen.
Point is, people take the length of post-production for granted, as if it's an irrefutable guarantee of superior visuals come release day. Problem is, as you've just demonstrated, this rationale is usually backed up by using examples of
unrelated, completed movies (known quantity), to confirm the outcome of an unreleased, unfinished movie (unknown quantity). In other words, the examples you used ain't Ant-Man, thus they can't be used to say that Ant-Man will look excellent when it opens. Different productions, different goals, different needs, different everything. Aside from that, for every example you can provide of a perceived improvement, there are at least as many showing dodgy CGI that was either equivalent to or worse than their trailers (Van Helsing, Mummy Returns, AngHulk, Crystal Skull; pick your poison), or in the case of TASM, completely different in some areas. Anything can change in post-production, for better or worse. If the studio sees fit to release footage advertising their movie to the world, then that says that they're prepared for all of the baggage and commentary that comes with it, good or bad. Bottom line - you can't discredit someone's impression of trailer footage based on an assumption.
For the record, I like what I've seen on the trailer so far, CGI and all.