The general consensus seems to be that a mash pH of 5.2-5.6 is desired. Beersmith even says it's the difference between a good beer and a truly great one.
Most mention trouble with too high of a pH, but I'm wondering, what are the impacts of a much lower pH, like 4.5 or even 4.0?
I ask because adjusting the pH to make the environment favor lactobacillus using acidulated malt would be a pre-mash way of lowering the pH (rather than a post-mash method like adding phosphoric or lactic acid). I would prefer to use the acid malt, but it is pre-mash, so would like to understand the consequences.
At least according to this experiment, there's not much negative impact, but it's hardly definitive.