You have to make both immigration and security checks, but overall still doable if your incoming flight is on time.
Recently had a connection AB to UA and yet managed to get my boarding passes printed at UA desk.

TXL is small and taxi time upon landing really short.
Not sure about the luggage question sorry. I'm always with a carry-on.

Here's the map: http://www.berlin-airport.de/de/_dok...inal-de-en.pdf