this, 1000 times this....you're comparing a 10 man BH (where shadow priests dominate due to aoe) to a 25 man Baelroc (where you're probably assigned to soaking a crystal, thus not attacking for 6s and potentially letting dots drop). Your actual dps numbers are irrelevant since the fights are so different. As for the DI uptime/actual benefit gained, I think you're either just misunderstanding how the buff works or the warlock in the 25 man was just absolutely terrible.
We really can't give you a definitive answer unless you give us logs.