If you like the music, it seems that the only practical thing to do is to seek out the Guitar Hero 3 rip on file-sharing networks.
Unfortunately, the only legal way to buy the album in high quality format is to buy Guitar Hero 3. If you own the GH3 game, then you can legally possess the tracks from Guitar Hero 3, but not if you own the Metallica CD, however. The specific mix/versions are considered independent, so far as I know. This means that owning the Metallica CD, which has very different mix/version of the songs, will not give you a right to possess the tracks from Guitar Hero 3.
Of course, the above assumes you are technically interested in being legal. I think that, sort of, anyone that has bought the CD has a right on ethical grounds to download the Guitar Hero 3 tracks from the standpoint of deserving a usable product(the CD is not usable on any real hi-fi system) for your money. But then again, Guitar Hero's producers worked to make the mix from Guitar Hero 3, so you would not be compensating them for their work if you did not buy the Guitar Hero 3 game. The only way to satisfy every point is to buy the game.
You know what? It's absolutely ridiculous that this situation exists and has to be considered!
BTW, I just bought the Mobile Fidelity mastered Kill 'Em All on LP. The sound quality is incredible - very realistic sounding. Very enjoyable.
-Chris