You want GTA 5 settings? You got GTA 5 settings. YouTuber wiliextreme has been through them all, testing out the extremes, the high and lows, and cataloguing them in video form. As usual the difference between high and ultra is so little you have to wonder if it's worth the extra load (but obviously it is, it's ultra. The frame rate'll be fine). There's also a mildly terrifying demonstration of nine different shadow options that doesn't even cover all the potential combinations.

Textures

Shaders

Shadows

Reflections

Water quality

Water quality 2

Grass quality

Grass quality 2

Distance scaling