(Post author here.) Curious to hear more details about your workload, because a 5+-year-old Fermi would truly be hard pressed to outperform Maxwell or even a Kepler K40, let alone Pascal.
Moving from a GTX 480 to a Kepler or Maxwell card, the numbers go up, but not the performance. I might have a corner case, but before investing in new hardware, I would want to benchmark first and not blindly follow the numbers.