Over-pressure masks don't change the dead-air space issue and the need to adequately ventilate that increased volume. Over-pressurized masks are especially helpful in contaminated environments (like PSD and habite based hyperbaric welding). Depending on the diver's position in the water, overpressurization can encourage greater lung inflation, but that doesn't improve ventilation when the divers shallow-breaths at the higher lung inflation.
My experiences would suggest otherwise; and I am well aware that OP masks are helpful in contaminated environments -- probably why I don't put the "hep" in hepatitis, from work, over the years, in harbors.
Venting air visibly escapes around the OP FFM skirt, enough for the manufacturer to require that it be mounted over a hood, rather than under -- certainly a sufficient flow to exchange a scant 0.5 liter inner volume; and my only headache-inducing experiences, related to potential CO2 accumulation, etc, stemmed only from poorly-tuned on-demand masks in the past . . .