cnet.com tested the iPhone 5 in a freezer, a fish tank and an oven when it first came out.
Baked for 5 mins or so at around 250º, the iPhone 5 did just fine. Don't know about the 5s or any other model (I'd think the 5c might start melting).
But as another poster mentioned, the issue here is NOT heat it's moisture. Liquid kills electronics. Take it in to a sauna or a place with high humidity and leave it there for a while and moisture collects. Get enough moisture collected and you have water, which is a liquid and will damage your device.
So, yeah, your device would probably survive the heat. But it probably would NOT survive the moisture.