Well, in summary: the orig. poster is concerned about a tuning variance of 5hz. I'm willing to bet that once the enclosure is built it'll be atleast 1 full-cycle off intended tuning.
I'm also going to say that the output of a generic 4th order ported will not be drastically changed for a 5 cycle variance, let alone the NFR, or 3db down point. Efficiency will not swing very far up the frequency band.
Furthermore, tuning most ported subbass drivers below 40-45hz in a basic helmholtz will negate whatver midbass possibilities that the driver may have had. By the time the driver response rolls off, it'll be anywhere from 3-12 db down by 80hz. At that point, Your mid's should have well 'kicked in.'
IMO, strong dedicated midbass drivers are a necessity in "good" reproduction in-car. However, I'm not a proponent of hpf'ing a sub driver nearly that high. Even high Q sealeds should be high passed at no more then the 60hz region.... assuming 12db/octave.
Just my aural opinion. That's why I said, IMO above.
SQ is subjective, output is tangible.
-Nick