Briggs, et al., 2004 p. 115 "Infusion mashes are best carried out at pH 5.2-5.4 (mash temperature), and so will give cooled worts with pH values of about 5.5-5.8. It has been recommended that decoction mashes should not give worts with pH values less than 5.5"
Kunze 184.108.40.206.3 gives pH 5.5-5.6 (room temperature, we must assume) as optimal as regards enzymes, but 220.127.116.11.8 advises reduction to 5.2 (again, we assume room temperature) to improve flavor stability, primarily by inhibiting LOX, which is of course less relevant with higher kilned malt or particular mash programs. Sureley such a low mash pH is problematic with regard to protein coagulation, hop utilization and other functions. Kunze is complicated. Again, we probably need to reassess.