Out of curiosity, just how much of 4K video will you be able to take on base 16Gb model straight out of the box?
The really cheeky answer is none as it's disabled out of the box.
Joking aside, I've got a few friends with the 6s, regular folk who don't visit tech blogs, I've asked to check their phones and on every one it is still disabled (because they don' know, not because they don't want it). Would be interesting to see what percentage have enabled the feature.