It's only a wireless N device, it doesn't have AC capability so if you have devices that make use of AC on the 5Ghz band they will be noticeably slower than with the Superhub 2ac which is of course AC enabled.
If I set the SSID's and security keys on both the extra router and SuperHub2 AC will they roam, i.e. will the wireless devices switch between extra router and SuperHub2 AC automatically when moving from room to room?
Yes your devices should indeed roam. You may want to experiment with Wi-Fi channels because I think I'm right in saying that the whole setup will work best if the hub and the router are broadcasting on different channels e.g 1 & 11 even though they will be using the same SSIDs.