This is called "personal DPS". However there's this other thing called "group DPS". While your pDPS will go down by pulling more (b/c slightly more Flash), the gDPS will go up (b/c more AOE from everyone). What this means is that regardless of tank choice, it's always better to pull big.
e: Sorry maybe I've misunderstood this. CoS works on any mob hit, so the more you pull the more damage you'll get out of it. I thiiiink what you're saying is that Flash isn't enough to keep hate? Or you're missing targets? In which case this could either be a problem w/ your weapon or your aim, because Flash is pretty generous with what it hits. Your defensive CDs are also really effective on big pulls, Bulwark pretty much only shines on big trash pulls, Hallowed means you don't need to be healed, Rampart/Sentinel both give you flat reductions in damage, Awareness even gets use since you have a higher chance of being crit. It's mainly just about trying, and trying again, until you get it right.
You don't need to worry about pulling big until 50~60, and only with appropriate gear. If you don't feel comfortable doing big pulls 'til you're 60 w/ decent gear + wep no one will really hold it against you. But if you feel confident in pulling more as you level up then you'll have an easier time when you reach 60.
