I'm not sure I'd limit it to 10-20%, there are some classes of things that are hugely different... I find that making my own stock/broth is at least twice or three times as good as pre-prepared, and even among pre-prepared there's a huge variation. This becomes much more pronounced when used as a sauce base.

I find some things I don't notice as much difference (cream, butter) as I will others (milk, eggs)... There are other things that are a matter of taste (grain, grass, grass-finished beef). I wish, for that matter, that more labels contained what they were fed especially for milk and eggs, as the difference between brands/containers and locations is huge.

I try to cook at least once a week, and sometimes that means spending a day making broth/stock (mushroom is an absolute fav, but the most expensive where I live). I find that there are just a few things that make a huge difference, and others not as much... others still are a matter of preference. I also find that mouth feel is as personal as anything else in terms of enjoying a particular food.