I used a 500,000 BTU weed burner last fall and the results were excellent. Most of the cat-tails around my pond were hand pulled, (ugh!) but one stand in particular was about 40 feet long and 10 feet wide running from shoreline to 6 feet of water. There were also willows in this stand. This stand had been the original stand which was there when the pond was put in in 1985 so they were rooted deeply so pulling just wasn't an option. I drew the water down by bottom siphoning and by the end of summer the water level had dropped 3 feet. When the willows wilted from frost I went out with the torch and burned them to the (reduced) water level. To date none have returned. It's interesting because the cat-tails which I pulled continue to sprout anew, not like they were, but they continue sprouting. The torched section is sprout free. Torching gets my vote.
One word of caution however, I live in a heavily wooded area and a large uncontrolled burn could easily be had with a torch putting out 500,000 BTU. I ignited a small section upwind and let it burn off the tops, slowly proceeding upwind until the tops were all burned away. Then I used the power of the gas torch to burn the unburned stalks to the water line without any fear of a run away blaze.

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