Are you boosting the highs in the mix, or at the amp sim? In the mix, with a sound like that you shouldn't need to add high end. If you want the guitars to sound thin like they do, you just need to get rid of the low end.
If you are going for the dime sound, you have too much gain. That adds to the fatigue as well. If you need more sustain than you can get with the gain turned down, use a compressor/sustainer to give you the sustain will keeping your gain level in the crunchy zone instead of the fuzzy zone.