A:Answer Kpfasho,
Yes, a GTX 1070-1080 would fit, though the power supply would more than likely need to be upgraded to support the required power connectors. As for the CPU bottlenecking the cards -- more than likely you would have a bottleneck with the current CPU if coupled with such a high-end card, though it wouldn't be as noticeable if gaming at a high resolution with high settings.
Thank you,