I've been mining an N Factor 13 coin as of late, and my 780 went from 10.7 to 16.0 kH/s with -L 3.
I'm doing the same (zcc) and lookup gap had no effect for me on my 660 (2GB), it stayed at 10.0 kH/s.
Here's my oversimplified take on lookup gap:
An increased lookup gap is virtually giving the GPU more VRAM to play with, but that only helps if the GPU was bottlenecked by the amount of VRAM in the first place so it doesn't help a thing if the GPU was already sweating to get the job done.
So an increased lookup gap with my mediocre GPU and mediocre VRAM amount, at:
N 13 had no effect (GPU wasn't bottlenecked by VRAM);
N 14 had a 30% performance increase;
N 15 had a 100% performance increase because the memory bottleneck is the worst here.