Without it (and shadow libs), I would get 4k40fps avg/38fps lower percentile, all other settings at the High preset in the benchmark. With it and shadow libs added to the bench, I was hitting as low as 4k24fps/4k20fps lower percentile during the combat section. Shadow libs on its own was about 5-7fps hit, VXAO was double that. I use a 1080 Ti at +70/+440. If you use the dr daxxy .dll+command line method, you can test it in the bench, but good luck trying to get it working in the demo. Keep in mind, the benchmark is not representative of the game at all (a bunch of assets are cut out, and most of the high features (hairworks, tessellation) were not implemented correctly).
On its own, it might be the single most intensive graphics feature in the game. Pick any other 2-3 Nvidia features, including shadow libs, and put together they might be less intensive on average than VXAO was on its own. This could change in the final game next week/post launch in a patch.