Well, I fell for that benchmark too, when I had a quick glance at the performance of Mordor.
You will get quite a bit lower fps ingame. Also it seems that not as much textures are getting loaded, making VRAM-consumption somewhat less than when you're actually playing.
I played with everything set to ultra (textures too) and resolution set to 150 % and then the game really starts to eat VRAM. I have 4 Gigs on my R9 290X and those are just not enough. I get small hitches once in a while - which are not really game-breaking but quite annoying. Those hitches are also there when I'm using native resolution.
Also, there are some effects like the rain which tank my framerate quite a bit making it go as low as 30.
I'm not sure what they did with the textures, they don't look like they should be so heavy on memory in any way.
There's an comprison between the two settings
here, it's in german, but you can click the pics anyways. Ultra-Textures use more than 5 Gigs on a GTX Titan (classic), so the recommendations for the game are not false in any way. The textures used in those pics where placeholders though, "ultra" do look nicer. There's an update to that article coming up.
You can enable Ultra-Textures by going to the DLC-Tap and then clicking HD-Content or Ultra-Content (or something like that). It should be another ~4 Gig Download.
[EDIT:] Oops, I got ninja'd with that link... anyways, those pics are not what Ultraa-Textures look like, these are placeholders.