My brew rig is a 10 gallon keggle gravity HERMS rig. I replaced my BKs 6" propane banjo burner with a 23 jet NG wok burner trimmed down to 12 jets, and I'm still piddling with burner height. In the old setup the burner was too close to the pot (about 4 inches), and the burner shroud didn't allow sufficient air flow, so I cut the shroud off and lowered it 1.5 inches with L brackets. We'll see how it works this coming Sunday.
My original concern with the setup was that, although it worked, I was getting way too much carbonization on the bottom of the kettle (lamp black, and therefore carbon monoxide too). By lowering it down I have allowed more air flow above the burner. In the test run it brought 1 gallon of water in the BK from RT to boil in 5 minutes.
I operated the propane burner with a regulator, and I had settings marked on its dial for heat, standby, and roll. I don't have any sort of repeatable control on the NG burner except for the noise it makes. I think of its settings as Whisper, Hiss and Roar. I use Roar to heat the wort to a boil, Whisper as it comes to a boil (to prevent boil over during the out-gassing phase that I call the "Foam Wait", which is usually 5 minutes), and Hiss to maintain a boil during the boil phase.
I tested the burner before I moddded the burner stand, and found that flame height in the Roar setting was about 7 inches. I feel that the ideal distance from burner tip to pot bottom is somewhere in the 6-7 inch range.
If this modification doesn't work out I'm going to find a sheet metal shop with a rolling mill and get them to make me a 12 inch diameter by 8 inch high cylinder of 16 ga mild steel and go from there.