[H]ard|OCP took a look into the effect on performance the gigantic high resolution texture pack has on system performance in their latest article.  For those who want the answer immediately, the largest amount of VRAM they saw utilized was a hair over 5GB, in most cases more than double the usage that the default textures use.  This certainly suggests that those with 4GB cards should reconsider installing the texture pack and that a 6GB card shouldn't see performance impacts.  As for the performance deltas, well we can't provide spoilers for their entire review!

"Bethesda has released its official High Resolution Texture Pack DLC for Fallout 4. We will look at performance impact, VRAM capacity usage levels, and compare image quality to see if this High Resolution Texture Pack might be worthy of your bandwidth in actually improving the gameplay experience."

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