For scrypt based coins ccminer is set to only use 88% of the maximum video RAM in case of Windows and 98% in case of Linux by default:
device_backoff[i] = is_windows() ? 12 : 2;
It is there for safety so it won't crash for people who wish to mine on their main card.
One of the changes I always do before compiling is to change that value to 1 for Windows to allow it to use 99% of memory and if you don't have a monitor attached to your card and not running something that is using VRAM needlessly, it won't crash. Not even while autotuning.
I used that since cudaminer days so I have no idea about the hashrate difference anymore.
Now that we're entering the territory of cards with 3GB+ video memory it's worth noting that 32-bit ccminer can't utilize more than 3GB.
64-bit can utilize it all but 64-bit is also noticeably slower so for cards like the 970 it doesn't worth it but for the 980 Ti it
might - for algos that scale with memory amount (not access) like Scrypt-Jane.
Afaik, 32-bit softwares can actually utilize more than 3GB video memory but only if the data is also stored in the system RAM which is generally not optimal for mining.
Edit: 200 pages! Congrats and keep up the good work sp_ (sent a few beers your way).