So just to summarize your claim of 550k-600k "ST" Dmg:
1) Not a ST fight
2) 30% buff every min
3) Ignored Mechanics
4) LFR...
Back to your original question if your using LoTV:
0 pieces to 4 pieces = 9.89% dmg increase
0 pieces to 2 pieces = 3.38% dmg increase
2 pieces to 4 pieces = 6.29% dmg increase
Source: Howtopriest.com