You shouldn't get any soot with propane and the 10 tip burners at about 4" below your pots. I get some soot from the pilots if I overcrank them but no soot whatsoever from the burners at 4". May be different with NG.
The tips of the burners are 2" below the bottom of the pot. These low pressure burners really work well for me at this distance... Actually, I took a vid last weekend of my wife brewing... I took some good footage of the burners working. I'll try and post it my friend!
Lonnie of Brutus 10 fame swears by them.......He mounts them about 2.0" under the kettle. Yours sounds like it may be mounted a little far from the kettle.
Start by removing all the shields, even the Blichmann orange ones and repeat the test with your current height. Once you have this data, you'll know where you need to go.
I use no shields whatsoever. With the frame, you don't need to protect the ball valve as the front member does that for you by default. Like I said before, I removed my heat shields as they were causing more problems then they were worth. Don't overthink it and try to change only one variable at a time.