- You can't double strike on attack skills (except duals).
- Every weapon is assumed to be customized, so it's irrelevant to the damage comparison.
- You base your numbers solely upon max dagger damage = wrong.
- Vampiric mods are also irrelevant.
The (average!) damage gained from the 15^X mod (assuming its req is met throughout an entire chain) would amount to...
Average base dagger damage: ( (7+17)/2= ) 12
Average damage gained per hit: ( 12*0.15= ) 1.8
Do note that this does not take critical hits into account.
Your example chain is a bit odd, if not impossible (3 attack skills before the first Dual?), so I'm going to use a standard instagib Offhand-Dual-Offhand-Dual build instead.
Of course, there's a large chance to critical. Going by these stats:
12+1+1 Dagger Mastery (19.6% base chance to crit)
12+1 Critical Strikes (13% additional chance to crit)
so roughly 1 in 3 attacks would crit. As an instagib chain hits 6 times, this would result in 4 'average' hits and 2 criticals:
( (1.8*4)+(2.55*2)= )
12.3 extra damage from the 15^X mod alone.
There's a factor I haven't taken into account: if a Dual crits, it always seems to affect both hits. I haven't found any definitive info on this, just my personal experience. I'm no calc head, but I like to have all the relevant info, so if any one else feels the need to crunch some numbers they are free to do so 