You count Tharne's goggles as +8 damage per swing.
In fact, against Harry, they are (in a full raid group) +3.667 damage per swing.
This is because Tharne's do not add 8 damage when you fail to overcome fortificaiton (50% of the time), and also they add no damage when you have Harry's aggro.
As Harry has completely random aggro (i.e. he doesn't attack the person doing the most damage), you have aggro 1/12th of the time in a full raid, and so 1/12th of the time you don't get the damage.
As having aggro and failing to overcome fortificaiton are independant events, the final figure is 1/2 x 11/12 x 8 = 3+2/3.