Well, I don't know how much you're trying to boil, but I use the Bayou Classic SQ-14, and I can get 5 gallons of water from ambient to strike temperature in a half-hour. I also use a bit of a dodge to cut that time. I use a 4 gal. pot to do my preheat water (about 3 gal), and when the MLT is preheating, I use that same pot to heat another 3 gal. That then goes in the 7.5 gal boil pot along with the other 2 gallons of cold water to make up my strike water. That 5 gallons is already hot enough so that it will cut the time to strike temperature down to about 20 minutes. The additional advantage is that I'm using cheap bulk propane in the house to heat the water as much as possible.
As for going to a bigger burner with your keggle, from what I've read on a few other threads, I suspect that a bigger, higher output burner is just going to send more heat as waste up the sides of the keggle. The metal will only transfer heat so fast.
“Malt does more than Milton can / To justify God’s ways to man”
-A. E. Housman (1859–1936). A Shropshire Lad , 1896.