--intensity|-I Intensity of GPU scanning (d or -10 -> 10, default: d to maintain desktop interactivity)
and what does that mean? how does it affect miner output?
GPU-README (yeah another of the files people don't read) INTENSITY INFORMATION:
Intensity correlates with the size of work being submitted at any one time to a GPU. The higher the number the larger the size of work. Generally speaking finding an optimal value rather than the highest value is the correct approach as hash rate rises up to a point with higher intensities but above that, the device may be very slow to return responses, or produce errors.
NOTE: Running BTC intensities above 9 with current hardware is likely to only diminish return performance even if the hash rate might appear better. A good starting baseline intensity to try on dedicated miners is 9. 11 is the upper limit for intensity while BTC mining, if the GPU_USE_SYNC_OBJECTS variable is set (see FAQ). The upper limit for sha256 mining is 14 and 20 for scrypt.