Aerocool KCAS 600W. Hmm... Those have a reputation for
very low quality. Even on Tom's Hardware PSU tier list, all Aerocool PSUs list on either tier 4 or untiered; tier 1 being the highest quality:
http://www.tomshardware.com/forum/id-2547993/psu-tier-list.html
I could not locate a performance review of the KCAS line from a single source that I know and trust.
In my opinion, once you upgrade beyond the power consumption of a GTX 970 / 1070, you can no longer afford to rely on the specs of a lower mid tier PSU. With your power supply's performance being an unknown variable in your problem, I suggest that you upgrade it. Below is my recommendation. Other than the power supply, I'm out of ideas of what could cause your FPS to decrease from a GTX 960 to 980 Ti upgrade. Assuming that your resolution has remained the same with these two cards, I still say that a bottleneck is extremely unlikely to be the cause.
One other thing that I'd like you to check: Even though it's unlikely for a i3 4160 to be overheated, especially with such relatively low usage, please check the temperatures after gaming for 5 minutes. Use the HWMonitor utility:
http://www.cpuid.com/downloads/hwmonitor/hwmonitor_1.31.exe
Re-check both the CPU and GPU temperatures and usage. Please respond back to this thread with all four figures. If they are within the expected range, then you're with welcome to see if anyone else offers a differing opinion, or simply follow my recommendation of upgrading your power supply. I can't guarantee that it'll resolve your problem, but I suggest that you do it regardless. The power supply is the absolute most critical part of your system. No exceptions.
EVGA G2 650
$89.99 ($79.99 after $10.00 rebate card)
https://www.newegg.com/Product/Product.aspx?Item=N82E16817438054
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