Rock/Metal guitarists tend to stay within the bounds of conventional "classical" harmony or blues-type playing. Very few players in those genres go beyond (Vernon Reid with Living Color being a great example.), and the faster they play, the more likely they are to stay within those harmonic constraints.
Some newer players in Death Metal (and elsewhere, I'm sure) are branching into more modern (i.e. after 1920) classical harmonies when soloing, but I still haven't heard the 90-mile-an-hour shred guitarists make that jump, outside of jazz players anyway. Mostly, when a player starts soloing using more modern harmonies that quickly, it tends to sound like random playing, or bebop. I think they avoid it as a result. Just my opinion, but it's backed by my experience with guitarists.
To hear shredding that isn't random-sounding, or conventional, I'd recommend Strapping Young Lad, Puya, Living Color, or anything with Al DiMeola in it.