You should be fine. I run my Phenom X4 980 at stock 3.7 and two 560 ti's and don't get a bottleneck
How do you know? Have you tried overclocking?
Just because you get good (or even great) performance doesn't mean there isn't a bottleneck. The key is to overclock until incremental gains in performance are no longer noticed with each subsequent overclock. I find it hard to believe that two high-end GPUs are performing at their maximum potential with a 980 black at stock, when my two 6970s (which just barely beat out your two GPUs, mind you) are bottlenecked by my Sandy Bridge up to 4.20GHz.