The tax idea doesn't make sense. It's not likely to happen. Corn is already subsidized and used to sweeten soda.
The idea that discouraging people from drinking diet soda is going to stop them from binging on a box of chips ahoy or fig newtons is stupid. Does soda stimulate the appetite? The miniscule amount of caffeine would suppress the appetite, if anything, and the liquid in the stomach would make you full sooner. Is not drinking soda gonna stop people from eating twinkies or a bag of doritos? No.
Flat taxes put more burden on the lower classes. The majority of us can afford whatever stupid tax they decide to levy; I feel bad for the poor people who are already having a hard time scraping together enough for food, clothing, gas and rent. The rest of us will have to listen to their kids whining at the store because they can't have soda due to some stupid tax they can't afford.
Whether or not you can afford another tax, it's not going to happen and it wouldn't help with anything except raising tax revenue.