I think if people knew how to buy seasonal ingredients, and not splash out on the big brand ingredients, they would put their money a lot further. 15% would be helpful if it included unprepared meats, milk, fruit and veges. I don't think it would be very hard to administer because as Phil said, the supermarket systems are already running software build for Australia, which has GST off food?
I think it would be pretty clearly defined too, I don't think a preprepared salad should be GST free, because it's preprepared, I think GST should be left to whole foods, of course they can be cleaned and bagged.
It's fairly easy to take a calculator and a shopping list to the supermarket, and just buy whole foods and cook.
I think if you don't have enough time to cook and eat a good meal, you have your priorities mixed up.
That said, I think that the 15% discount would be quickly absorbed by supermarkets, I'm sure they would rather sell processed foods anyway, they don't go off after a week!