CAI , Exhaust, or Programmer

I have a 2003 Silverado 1500 with the 5.3L. It has 125,000 miles and I'm trying to decide what to upgrade first. I want to improve my mpg mostly. How much of an increase can CAI's really give you. I was looking at getting a K&N because most people seem to get those and they seem happy with them. I saw on a review where a guy improved his miles per tank from 320 to 400 after the CAI, is that possible seems a little too much for just a CAI. Also is a throttle body spacer worth it? What exactly does it change?

Also how much does exhaust improve your mpg if you drive normal. I know driving habits can change after exhaust is installed and more than likely I would fall in that category so I'm thinking this is going to have to wait. I've also read getting headers can help. Can someone explain how this can help, I'm not very knowledgeable with this stuff but trying to learn.

For the programmers I was looking at the Diablosport Predator. Everyone on here seems to like that, but how much does it really improve your performance and mpg.

For right now I want to just do one of these because I don't want to blow all my money on my truck. What do you guys suggest?

Thanks in advance!