at the moment the main problem of gamestudio is the map compiler. it doesn't work properly for polygon soups. once it works (hopefully soon) it will be much easier to do something like that in gamestudio. it also shouldn't be slower then. outdoor levels like in the video don't make use of portals or anything.

and like already has been said it can be done already with some workarounds (i have seen a video from david lancaster for example (he didn't show it on the forum yet) which i found much nicer than this one).