I'm amazingly lazy, but it'd be easy enough to verify. Take a digital recorder. Hook it to the mac's optical out. Record a track. Transfer the track back to the mac. Invert it, and then sum it with the original track.
I'm not going to do the work, because I'm happy with the sound I get from my mac, but if you're concerned about the bit perfection of the mac, it's easy enough to test.