I'm saying the mfgr.'s btu rating is inaccurate for a homebrewer's purposes unless the burner's owner properly sizes their propane tank with their burner. Sure a 20# tank has the potential of 430k btu, but that 430k cannot be accessed to yield 185k btu.
A full 20# tank can produce 90k btu/hr at 70F. The btu declines as the temps decline. e.g. a full tank produces 65k btu at 40F. That's why I suggested a larger tank. a larger tank will yield a better vaporization (vapourization for you Brits) rate. Better vaporization rate = higher btu.
So, in short, the best any burner can achieve using a 20# tank as a fuel source, assuming 70F temps, is 90k btu. The rest of the btu rating number is just d!ck waving.
Here's another's explanation. I tried to search for industry data but couldn't easily find it and I'm too lazy to search any further.
http://answers.yahoo.com/question/index?qid=20090124214947AAxUOqm