I use NG, and am completely satisfied. Yes, it is a little slower than propane, but not that much. However, I use a larger burner than what you have:
At this size, with NG it puts out 90,000 btu, while the propane version puts out 92,000 btu. Not a significant difference in my book. I can bring 10 gallons to a boil in about 25 minutes. I have a 10' hose connecting it to the pipe outlet. I believe this is restricting my flow somewhat, but it was cheap (closeout NG conversion kit for a grill).
Burner positioning from the kettle is specified to maximize heat transfer. You need space for complete combustion, and the hottest part is the tip of the flame. The line that "Please note: Make SURE your pot is a minimum of 12 inches from the tips of the natural gas brass jets! They will melt your pot!!!!"
is total hyperbole.