The heat pump mode works exactly backwards to the air conditioning mode. In the cold season, the compressor makes heat that is exchanged internally and the outside condenser will be colder than the air around it. That is why they ice-up and require a defrost cycle.
Your first step should be to permanently fix the problem of water entering the unit.
The answer to your question is NO! Enclosing the outside unit would keep more cold air around the condenser reducing efficiency. However, there are certain variable like size of enclosure to consider.
You could actually increase effeciency during the daylight by building a glass enclosure around the condenser if it gets considerable sunlight in its location. The additional heat around the condenser with increase effeciency. However, the room would have to be about 12 foot by 12 foot in size to work. The room would trap heat energy from sunlight and aid in the condenser's operation. However, at night it would not help unless the room was built over large rocks that retained heat for long amounts of time.
Then in the summer, you want the opposite to be true. You would have to move the condenser to a shady cool area to aid in cooling. And in Dallas, thats a lot more important than heating. I was in Dallas last August. You need cooling more than heating!
To summarize all of this, the answer is NO!
You could however make sure that the condenser is cleaned of debris twice a year to improve operation of its heat exchanger.