What are the weather overlay capabilities of these units? Can you overlay
a radar map on top of your airspace map? Across a route you are
navigating?

Think of the presentation you see when you go to aviationweather.gov.
That is pretty much what you get on the 496.

Depending on which of the two available XM WS services you subscribe to,
echo cloud tops and direction of movement, lightning, winds aloft,
surface analysis, severe weather, SIGMETS/AIRMETS, satelite mosic and
NEXRAD.

I used the winds aloft function on my return from OSH to take advantage
of an available 15 kt tailwind at 6000 ft.