You shouldn't have to rely on random stuff (doom nova, spirit eruption and the likes) for the imps. If you're missing damage on them and the doom nova covers it up, then you need to redefine roles.
https://www.warcraftlogs.com/reports...s=1795&wipes=1
Why is Nasks doing double the imp damage compared to Coppelia? Surely the latter has lesser gear, but nowhere enough to warrant such big difference. Also if you go log by log, you'll see them having huge variations in damage done to the imps. That means that you can't get consistent tries in. Also, I shouldn't need to mention that the Demo lock doing less imp damage than the Destro is a travesty. Why does he only have 25 Chaos Waves and 25 Cataclysm casts in 29 tries? Additionally, your ret should be doing much more damage on them. Understand he was subbed in and out so he wouldn't be able to compare, but for starters he should be playing Final Verdict, so that when imps are about to spawn, he already has a FV buff up and holy power for a divine storm, so he fires off a buffed divine storm and an unbuffed one, for tons of damage. Rallex can also do some serious imp damage with double Chi Torpedo and Chi Burst.
As you can see you have a lot of classes with the appropriate tools to deal with the Imps, so you shouldn't gimp mages and the likes just to deal with them.
Infernals are definitely not a priority. Besides, you have boomkins, shadow priests (not as much if they're playing CoP but still - Cascade), a hunter and a destro lock. Those classes alone should be able to deal with them (hunters for the extra damage on >80% health targets, shadow priests, unholy DKS and boomkins with DoTs and Cascade/Starfall and destro lock to snipe Shadowburns for embers), without really focusing them down.