The reason for ram requirement is that consoles today can utlize more ram than before it also means textures can be more detailed for the consoles today etc..all arising demands for pc.
Played shadow and cant say I found the game demanding.
the below test shows ultra textures fps from NH.
Meeting the VRAM requirement for that particular title wouldn't necessarily improve avg frame rate compared to the same card with lower VRAM.
As I understand it, this is intended to reduce stutter and hitching on Ultra with the optional texture pack, using the in-game DSR/downsampling setting (e.g., renders at 1440p or 4k, then downsamples for 1080p display). Under those settings, some have reported VRAM usage up to 5.9GB on Titan Black as well as reduced hitching.
When space runs out in VRAM, the old texture data needs to be switched out and new textures loaded up from your drive and/or system RAM into the VRAM. "Hitching" occurs when a slight pause results from your texture data getting switched in/out of VRAM, loading up from the drive, etc.
If you don't have enough VRAM, you could have over 60fps avg frame rate, but still experience hitching when your GPU has to pause to wait for new data to be loaded.