g_ether over the A20's musb (Mentor USB) interface is... very broken.
No doubt. But IME that tends to apply to a large proportion of USB thingies.
so what you are probably running into is the musb driver going "ha! you plugged in a *what*?? pffh i have no idea what speed that is so let's just assume it's USB 1.0 mkaay?"
I don't know what's the source of the 10MB/s performance, but note that 10MB/s is still 7 times faster than the theoretical upper bound of USB1, so it's definitely not putting itself in USB1 mode.
a proper USB2 host controller should have none of these difficulties.
Let's hope so,
Stefan