The 820 has the all of the navigation functionality but it has some properties that make it less suitable for navigation than the larger units. The 1030 is better for navigation than the 1000 (mostly because of the screen).

1- it seems like the CPU on the 820 is a bit slow. It's small size means it has a small battery. It appears the CPU runs a bit slower than the larger units to conserve power. This is really only an issue for navigation (navigation works better with a faster CPU).

2- the screen on the 820 is smaller and square. An elongated screen (like the larger units have) means you can see more of what is ahead on the map. A bigger screen is also better for navigation.

Everything requires compromises/trade-offs.

I am mostly interested in navigation, which means my focus is on how well that use is supported.