If someone has a photo of Xastir running "full screen" and can post a picture I would appreciate it and would give it a second look
Here's a screenshot of Xastir on my Mac:https://docs.google.com/file/d/0B8dALljET_kMQkthU1BwY0pYTjA/
That's on a 15" Macbook Pro, non Retina. You can see the buttons are very small and I think would be unreadable on a 7" display. That, along with most people having a difficult time getting decent off-line maps, would make it a deal breaker to me.
I've used APRS Droid (Android) for messaging using the APRS-IS interface and it performs very well. Now that they have options for Bluetooth TNCs and AFSK encode/decoding, it might do what you want. Downside is it uses Google maps, which come in over your cellular data. I know that Google maps has a way to download sections of maps for offline use, but I don't know if they carry over to APRS Droid and other applications.