Best guess: roll-up of the experimental branch patches so far?
It's a couple of smaller .pak files (haven't extracted them, but typically settings, functionality loops - things like how to render, rewrites of some routines, script tweaks). And a gigantic 900mb pak (could guess that this is the recompiled stuff).
I'll.. repeat the suggestion to gog-staff again. Create a dialogue with HG about offering some feedback collection on the new patches. Allowing HG to push the changes they put out further. And release these patches on an experimental branch, separate from the stable branch. Have them document the actual changes for each build. And gather feedback for the new builds specifically. Instead of having this "overall impression seems to improve" thing that no doubt their internal q&a is doing.
You can offer HG a valuable service here, in having "real world" feedback, structured specifically in response to each individual patch. And it wouldn't take much effort on your part. Just as each of us won't really have to individually make a huge effort either.
It's also useful for you to avoid refunds (and for HG in the same way). You can have the stable branch available with the requirements documented (sse3, gtx 480, etc.). And then refer people who wish to refund to try the experimental branch.
Meanwhile, gameflow tweaks could be tested towards real people, rather than just their internal reference groups.