Opinions on what I've come up with for a 12 V-to-5 V automotive circuit?
You specified a current draw of 800mA. The voltage drop is 12-5=7V thus the regulator will consume 0.8*7=5.6 Watts. That will likely burn your regulator from the board. 

Even if you take a different type and a heat sink, you don't want a big heat source in your car.

I strongly suggest you use a ready-made 5V switching regulator. They come in a bit larger TO220 housing.