I played around with my 6950s mining LTC, reducing the mem clock from 1250 (or whatever the default was) to 300 and it made no discernible difference to hash rate or power draw (using a Watts Clever meter).
After discovering that I just put it back up to stock speeds.
Maybe different cards respond differently??
The power draw and the temperatures should be lower with VRAM at 300 MHz. Open GPU-Z to check the actual clocks.