Just a few thoughts. I've seen several articles lately stating incorrectly that mash pH is to be taken at mash temp. I honestly don't see where all the confusion comes from. The textbooks all clearly state that the desired pH is 5.2 to 5.5 in the cooled sample, and that the standard is room temp pH because when the optimum was first determined measurement could only be made at room temp. Some confusion seems to come from the red herring of the supposed -0.35 displacement. This seems to be based on a single series of tests reported in DeClerck that have proven unrepeatable. The actual displacement seems closer to -0.2. But again, the original room temp standard has always been maintained for citations in the literature. Maybe there's mass confusion about ATC. But of course you also want to take your reading at ROOM temp because, well, it's nice not to kill a $60 electrode in just a few batches.
Anywho, all I can tell is that the prescribed pH always has been 5.2 to 5.5 at ROOM temp, and mass confusion has only recently set in. I suspect one erroneous article appeared and its effects have reverberated around.
All that said, experience should tell us some things: Brewing processes and beer quality are clearly optimized as mash pH centers around 5.4; and pH as low as 5.2 was only helpful in the days of protein rests and moderately modified malt. 5.8 to 6.0 range is the DI pH of malt which has always been considered unacceptble, and I don't see how they get that number for an adjusted mash, even with the mythical 0.35 displacement.
I had thought this was all pretty clear, and any article to the contrary would have just brought a correction notice in the next issue. The confusion is confusing. Sorry for rambling it's getting late.