I've been carrying a Heatsheet blanket and an extra-large orange trash bag and have been confident those two items would be adequate in a survival situation. I have considered one of those AMK bivies, however I changed my mind and kept the Heatsheet blanket. Here's why :

One day at the end of a dayhike, a lady went into anaphylactic shock after a sting. It was raining, she was cold, her speech was slurred, it was a real emergency. I asked for a survival blanket "or something", then one guy pulled out something that looked like an AMK blanket, and handed it to me. I went "perfect" and opened it up : it was a bivy. No time to slit it open, so I had to manage with this narrow bivy to cover the patient, knowing the reflective layer was inside. I wish I had my pack nearby but all went so fast, I felt it was better for me to care for her (raise her legs, watch her head, etc.) Since that event, I chose to go with the Heatsheet blanket.

You seem to pretty well know what you're talking about, my friend. I would just not worry too too much if I were you. Like Dale Carnegie says, stop worrying and start living