all original textures, yes. All Microsoft (yes, this is done by the Xbox backwards compatibility team, not rockstar) has done is increase the resolution and apply 16x anisotropic filtering (dont ask me what that means lol, I'm not really a tech guy).
And I think you mean Rockstar San Diego, not North. Anyways, I agree, this holds up quite well.
My bad. I thought it was North. Rockstar San Diego, then.
Still, this shows how much effort they really put into those textures. If they look this good when resolution has been increased that much. Absolutely impressive.