In a way, per this guru3d review of Broadwell-E, you made a good choice on CPU since it is the cheapest of the Broadwell-E platform and getting the higher core models for 1440p would be a waste of money since more cores doesn't do much when gaming max detail (games become GPU limited).  

But in this guru3d review of GTX1080 SLI, in which you actually have both cards, the implication is that 1440p isn't high enough a resolution to utilize the graphics cards, that their scaling shines at 2160p (popularized 4K).  Oddly enough, they tried to conclude that at 1440p, the SLI'd GTX1080 was suffering a CPU bottleneck which kept the pair of cards from scaling properly.  But comparing the results in both reviews proves that the scaling of the cards were dependent on the resolution.  If the CPU was truly bottlenecking, then the change in resolution shouldn't have done anything, as if everything in the system is held back by slow processors.  High-end graphics cards need high resolutions to shine, that's just how it is.  Unless you play CPU-limited games, changing the CPU won't do much.  Your future potential upgrades are dependent on what you do, that's why there isn't a one-size-fits-all consistent simple answer.

Someday soon, say next year at this time, there will be single graphics card (GTX1180) that will be as fast as your GTX1080 SLI.  You could put two of them in your system, but don't scapegoat your CPU for why they don't scale, the problem is your resolution, you need to get into 4K for that.