I tried a lead film container years ago before TSA, 911 and all the rest. Prior to using it, my bag would sail through the scanner. The one and only time I used it, my bag went in, they stopped the belt, and studied things for a couple of minutes, or so.
30 seconds zipping through the machine, vs a couple of minutes of collecting x-rays (presumably), made it an easy decision to ditch the lead container. I don't know how much lead it takes to stop whatever the machines can produce over a few minutes, it's not an easy thing to test. OTH, I've had various rolls x-rayed in carry-ons, sometimes multiple times, off and on for 20-30 years, and have never had a problem.

For checked bags, if there is anything they have doubt about, they'll open the bag and look. I often have varous electronics and thick manuals in my checked bags, finding the little TSA greeting card in my bag on arrival is a pretty common thing. I don't think I'd trust the foil bags to checked bag x-ray. Consider the difference between those little bags and the vest your dentist puts on you for getting tooth x-rays.