Unlimited plenum was the 'cut a hole in your box like Mountune did' mod.
I'm going to butcher this explanation, but I'll give it a shot.
Less filter pressure would read as a higher number. During low-load driving conditions, filter pressure is going to be the atmospheric pressure (because airflow is dictated by the throttle plate), which is 14.7 psi at sea level. As you rapidly increase load and/or move from negative into positive pressure with the turbo, you create a low-pressure area in the intake tract. The more air that gets forced into/through the engine combined with the rise of engine speed causes the pressure to continue to drop until you close the throttle plate-allowing the intake tract to equalize with atmospheric pressure.
My filter pressure with the OEM box was roughly 14 psi at 6500 rpm with an airflow mass of 20.8 lbs/min. With the hole I was seeing 14.07 psi at 6500 rpm with an airflow mass of 20.88 lbs/min. Estimates of airflow mass on a performance engine are roughly around 9-11 hp (-ish, there are a lot of variables in that) per lb/min, that would technically confirm a gain of .72-.88hp, roughly (mathematically). So even though it is technically a gain, it's not one that's very measurable, definitely not enough that would be 'felt', and most likely not going to be able to offset the cost of a new lower box.
I'm going to butcher this explanation, but I'll give it a shot.
Less filter pressure would read as a higher number. During low-load driving conditions, filter pressure is going to be the atmospheric pressure (because airflow is dictated by the throttle plate), which is 14.7 psi at sea level. As you rapidly increase load and/or move from negative into positive pressure with the turbo, you create a low-pressure area in the intake tract. The more air that gets forced into/through the engine combined with the rise of engine speed causes the pressure to continue to drop until you close the throttle plate-allowing the intake tract to equalize with atmospheric pressure.
My filter pressure with the OEM box was roughly 14 psi at 6500 rpm with an airflow mass of 20.8 lbs/min. With the hole I was seeing 14.07 psi at 6500 rpm with an airflow mass of 20.88 lbs/min. Estimates of airflow mass on a performance engine are roughly around 9-11 hp (-ish, there are a lot of variables in that) per lb/min, that would technically confirm a gain of .72-.88hp, roughly (mathematically). So even though it is technically a gain, it's not one that's very measurable, definitely not enough that would be 'felt', and most likely not going to be able to offset the cost of a new lower box.