How much does Improved Shadow Bolt improve warlock dps?

How much does Improved Shadow Bolt improve warlock dps?

With 5/5 Improved Shadow Bolt (hereby known as "ISB"), on a crit the target gets a debuff called Shadow Vulnerability which increases shadow damage taken by 20%. This lasts 12 seconds or 4 charges, whichever comes first. Some facts about Improved Shadow Bolt:



• Hits which are resisted do not eat up a charge.
  
• The debuff is occassionally resisted separate to the damage spell itself (ie, the Shadow Bolt will still hit, but the Shadow Vulnerability won't go up).
  
• Crits will reset the counter to 4 charges.
  
• Can be consumed by any Shadow Spell such as Shadowburn and Death Coil.
  
• Since patch 1.11, DoTs no longer eat up the charges. However, DoTs still DO benefit from the +20% shadow damage as long as the effect is active.
  
• ISB slightly increases the value of each +1% crit you get from gear.
  

Math
To calculate how often Shadow Vulnerability will be up on the target: SV = (1 - (1-c)^4) where c is your crit rate.

Example: assuming raiding warlocks have the reasonable average crit rate of 15%, Shadow Vulnerability will be up (1 - (1-.15)^4) = 47.7% of the time.

Math
To calculate how much ISB increases warlock DPS (using the variable SV from above): SV * .20

Example: Using the above example where SV is up 47.7% of the time, ISB increases your average Shadow Bolt damage by (.477 * .20) = 9.6%.

Since SV also passively improves DoT damage, your increase in DPS will be even greater than 9.6%. The lower your crit rate to begin with, the BIGGER the increase from crit will be due to Imp SB. Conversely, if you already have a high crit rate, you will see a lower increase in damage from crit due to ISB.

Example: Going from 5%->6% crit gives you a 0.67% increase in damage from ISB. Going from 30%->31% crit gives you a 0.27% increase in damage from ISB.

While the value does go down, more +crit is still worthwhile to get. The value goes down slowly, the fact that it goes down at all isn't a big deal. The real moral of the story is that +crit is slightly more valuable than most people believe.