VanitySearch may not compute a good gridsize for your GPU, so make several tries using -g options in order to find best performances.

What is the best and most efficient way to determine and use the optimum gridsize for a GPU?

The author was asked similar question in past and he only recommend to try different number[1]. Personally i'd recommend you to see benchmark list created by DaveF[2] as reference. Of course there are general guide to choose total block/grid on CUDA[3-4], but i've no idea whether it can be applied to VanitySearch's gridsize.

[1] https://bitcointalk.org/index.php?topic=5112311.msg54144836#msg54144836
[2] https://bitcointalk.org/index.php?topic=5112311.msg50823897#msg50823897
[3] https://stackoverflow.com/a/9986748
[4] http://selkie.macalester.edu/csinparallel/modules/CUDAArchitecture/build/html/2-Findings/Findings.html