Unfortunately that video shows that the iMac is not fast enough to play the game at maximum resolution and settings. The YouTube video just shows the first 10 minutes of the game - which is entirely scripted. The video you see at the beginning 'Atlantic 1960', is just an AVI... the first bit of 'almost' actual gameplay comes after 8 minutes of the video, and even then it's still scripted with relatively little requirement from the GPU - and yet it is still jerky - well below 30 fps I'd say.
Having said that, if you drop a few settings, maybe down to the next resolution below native, you could have a playable Bioshock experience.
And I really do recommend playing it - I've played the whole thing through twice in the last week, much to my wife's bewilderment.