Long Tube Headers on a GM LS1

I bashed one of the catalytic converters on my 09 Silverado 2500 / 6.0L offroading a couple weeks ago, and must have broken up the guts because it started throwing a P0420 code yesterday. It will cost about $350 to just have a muffler shop weld in a new cat, but I'm wondering if it's worth going to aftermarket long tube headers and Y-pipe (keeping the cats). I'm looking at Kooks and Dynatech systems ($1500ish complete), but probably would keep the stock catback because I like quiet exhaust.

I could use some torque for towing, but otherwise I am happy with the performance. I'm not an aggressive driver for the most part, and I would really like to pick up a mpg or two if possible. Does anyone have any real world experience with actual gains from just adding a good set of headers? Dynatech claims up to 46 hp / 38 ft-lbs, which strikes me as absurd. No claims on mileage gains that I can find...

I know a cold air intake would help too, but I'm not really a fan of CAI's mostly because oiled filters will gum up the MAF sensor.