Tenths on the mic are gained...that is about it in most casses. Once you reach ~80% of the driver's Sd you will no longer gain SPL and may begin to loose SPL depending on the port and driver arrangement. Once you get over 100% of the driver's Sd the sub may begin to act "free air".
Best advice for daily is enough to where you wont get any port noise. A peak modled velocity of 60ft/sec or less in WinISD with maximum power is usually subsiquent in the real world (though velocity calculatons are usually innacureate).
Excess port area isnt always good either. More port area means faster roll off below tuning. Some HT set ups actually cut back port area to keep the driver from unloading below tuning so the infrasonic frequincies can be reached with out destroying the driver while keeping a flat respocne instead of a shelf responce.
