The gamecard says it supports only up to Win7, which I find a bit puzzling, especially as the other games on GOG that utilize ye olde Unreal engine run just fine in Win8 - at least I haven't come across any problems in Deus Ex or UT. Yet skimming the Steam forum, it really does sound like for some reason the game's performance may not be up to par in Win8. Can anybody confirm this? Does the old 'gold' version run any better?

Since someone mentioned the Direct3D 10 renderer in the release thread I also wondered if it really does alleviate the issue, although that one doesn't support the updated 'Classic' version.

I just want to make sure I won't be running into serious problems if I buy this.

Shortly after Windows 8 released I had done a few tests on different systems regarding the issue. At the time, any of the old Unreal engine games including Rune suffered from major slowdown issues, although they did work when using D3D10 renderers. (Such as the one you posted)

It looks like SirAlastor has made a post confirming that the D3D10 renderer does not work with the GoG version, but that the retail release is also included which can be used instead:
http://www.gog.com/forum/rune_classic/direct3d_10_renderer_gog_version/post3

I'm playing this on Win8 now. No performance issues for me so far. Just had to edit the.ini file to set the full screen resolution as the option in game does not show it on the list.

I'm using Windows 8.1 64bit, and performance is not ideal here (closer to 20fps than 30), with both games. The only thing which improves this for me is running the game in window mode, then it works 100% OK (as usual with Win8 it seems - lag in fullscreen, but games running better than ever in window mode, often without glitches contrary to Win7 in the same situation).
I've tried Kentie's D3D 10 renderer on Rune Gold, but it sutters, it may not be the same issue though [edit: found why, Vsync had to be disabled under Advanced options=>rendering=>Dx10 renderer]. GOG provides nGlide for Glide rendering, if you use this one with Rune Gold, it's running fine. So, here, it's either window mode for both games, or Glide for Gold. As Win8 emulates DDRAW, I've also tried disabling this in the advanced options, no luck.

I was about to ask the same thing about this game seeing gog has it now.

I ran into the same problems trying to run the game in windows 8.1

slow in fullscreen and warp speed in window mode.

8.1 x64 Quite a few crashes here and there using gog's version of classic.

resolution and fullscreen worked fine, every so often when it begins a precache it'll lose fullscreen and when i try to refocus on rune it'll crash.

so i save a lot and it works fine :)

EDIT: official 8.1x64 Nvidia drivers seem to have fixed the issue