Even if there's such additional data downloaded for the sake of Autopilot, it's not going to happen, in all likelihood, in real-time for a specific area as you actually drive through there. Any such data downloads will most likely happen - preferably over WiFi - while your car's snoring in your garage.
If you are on 3G and are considering an upgrade to LTE, there are many other reasons to fork out the $500. Coverage is better, Slacker loads much faster, maps are a bit better and so on. Just got the upgrade myself today and am loving it.