I've done side by side tests with no changes except memory and have found no difference between 1500mem and 1665mem. Seems equihash loves the core clocking tho. I've been reading this many places. You can even reduce your mem and it stays the same.
On my system/drivers/card overclocking modded vmemory gives additional speed. I checked this.
Overclocking the core without overvolting more than 1085 MHz make mining unstable.
And you tests memory with modded stripes?