For those that are using invocation, you're keeping the stacks up while you use invocation correct? I'm thinking of giving this a try on h elegon tonight. I can't see a reason to barrage before using it.
If this is the case, it appears to arcane mage community has found another way to stay at 6 charges and not clear stacks with barrage like blizzard wants.
Yes, with invocation you still keep the stacks if at all possible. Glyphed AE can help with this, especially on Elegon when you may lose them after pillars drop but you may or may not be wanting to pull threat of an add off the tank.
The problem with invocation haste on Elegon is needing mana for orbs. If you camp your missle procs well though, you should be able to get through without too much issue.