The GOG gamecard tells the game is compatible with Win8, but I can't play it in scaled fullscreen (i.e. with black borders):
Else it's a small window, else the game is stretched.
How to have a nicely scaled game?

Why wouldn't you play it in full resolution?

Scaling option of Catalyst driver is greyed since I've Win8.

Is this due to Win8 or due to using the monitor's Native Resolution? I recall Intel had the scaling disabled if running at native resolution, and I think ATI did that as well. Try changing the monitor's resolution to a non-native one and see if the scaling becomes enabled.

If this is a change introduced in Win8, feel free to ignore the above advice, but I do find it weird.

Yes I already tried this trick: the driver scaling option is no longer greyed in non-native, but it does nothing when coming back in native res and then launch the game. Yes I tried the option in 1024x768 game resolution.

Works fine in Windows 8.1

(catalyst 14.9 WHQL)