I assume it is 6% increase of ACTUAL dmg done by your bolts, otherwise it would be a shit bonus.

Funny thing is, 2 piece bonus is amazing for affliction locks, 4 piece bonus is amazing for destruction locks.  There are very few affliction locks in end game guilds.  Guilds usually keep one around for malediction or for certain fights where they come in handy, but they will never beat a destruction lock in DPS.  So the 2 piece bonus is kindof crap, why would a destruction lock ever cast corruption?  And only a fire lock, which is incredibly gear intensive, would ever use immolate.

Anyway, t6 gear is still amazing, too bad it's getting to that point where if you aren't on Illidan yet you will be very disappointed and feel unaccomplished once the expansion comes out.