In the United States, the max is 4W. The XTend is already running at 30dBm output power, so the max gain antenna you can add without any loss on the cable would be 6dBi.
When increasing your gain on an antenna, you are actually NOT increasing the output power. Rather, you are focusing the existing power into a more tightly controlled beam (much like a magnifying glass with the sun's rays).
If you are not in the U.S., then you will need to look up the maximum allowed output power and gain.