Check with the BBQ manufacture to see it a natural gas jet is available, the other question is NG at your house or you propane heated like many members?
I find 15,000 BTU's rather low in heating output besides adding the pressure that is reaching the burner will change your heat output by a large amount. You should deduct what heat passes past the sides of your boil pot not heating where you want it. I was thinking even with electric heating of 11,000 watts for 37,543 BTU's at 100% but using 95% as a true heating transfer of 35,665 BTU's directly into a liquid even with available power still on the lower end of the heating scale. Insulate the boil pot there is a large surface area that can be protected and converted into faster heating times. I'm using this as an example vs those propane units with numbers mentioned of 80,000 to 240,000 plus BTU units. Many members has stated a vast difference in propane burner BTU outputs for the same burner just different suppliers selling the same burner. A little "new math" or stretching the truth here? No one has done a under lab testing conditions reports on all these propane burners and their manufactured claim outputs including the propane PSI at the burner. Wind chill alone will sap the heat away that side burner has to offer. I have a Commercial CharBroil in stainless with a nice looking brass side burner, for up to a 12" frying pan it works great cooking outdoors, larger boil pot I say out of the question. I fry squid outdoors, Never In The House you'll never get the smell out. Trust me on this one this includes a Fry Grand Dad fryer.