Because there are no official comments her from GoG, i wil put a link to the Steam Forum:

http://steamcommunity.com/app/336610/discussions/0/

There are a lot of questions and answers, i.e. incompatibility with Win 8.1!!
Hope that will help, maybe someone can make this sticky for a while.

Looks like the Steam version has had several updates already - I've seen build 18704 mentioned, the GOG version is build 18700.

I have a fix for one of the issues right here, check this thread:
http://www.gog.com/forum/outcast/a_fix_for_lag_and_screen_mode_change_for_11

Ok, Stickys have disappeared at Steam forum, now there are release notes für different versions/patches with fixes. Hope we will get them soon too.

http://steamcommunity.com/app/336610/discussions/0/626329820860443987/

Cool, they seem to have fixed the insane performance hits and potential crashes during fullscreen effects (although for some reason they have listed them as different issues while it's most definitely the same one). That was my biggest gripe with Outcast 1.1 so far.