We've benchmarked GTA V on PC, but if you're wondering just what kind of difference moving the settings around actually makes, here's a very helpful comparison video.
wiliextreme's clip goes through everything from texture quality to shadows to MSAA to ambient occlusion, showing the same spot in the game and showcasing just how much better/worse everything looks with each change.
Here are a couple of slider examples (drag the arrow across to compare) taken from the video, which are great not just for their practical use, but as a reminder. I'd never noticed that a chainlink fence actually had a 1:1 shadow before, which is cool.
TEXTURE QUALITY: NORMAL vs VERY HIGH
SHADOW QUALITY: NORMAL vs VERY HIGH
You can watch the whole clip below: