Bottom line, you need a much bigger burner.
BTUs and BTUs per hour are often confused, especially since many burners report a BTU rating when they really mean BTUs per hour. A BTU is the amount of energy (heat) needed to raise 1 pound of water 1 degree Fahrenheit. As Wilco points out, it takes 1,178.6 BTUs to raise 1 gallon of water from 70F to boiling (at sea level anyway where water boils at 212F). That's 5893 BTUs to boil 5.5 gallons of water.
The Dark Star Burner 2.0 claims it is "loaded with 65,000 BTUs of brewing power". That really means it claims to output 65,000 BTUs per hour.
In theory then, the Dark Star 2.0 will boil 5.5 gal in under 6 minutes (5893 BTUs / 65,000 BTUs/hr * 60 min/hr = 5.43 minutes). Reality is nowhere near that fast, however, because not all of the heat produced by the burner goes into the water, the water is also losing heat, both by boil-off and to the air around the pot, and you don't often run the burner wide open (at 65,000 btu/hr) because doing so would just waste propane - see http://forum.northernbrewer.com/t/bg12-vs-bg14-burner/24552
There are a few actual boil times reported here http://forum.northernbrewer.com/t/lets-gather-data-btu-and-time-to-reach-boil/16767