aside from extended response lower tuning allows for:
lower vent velocity and less compresion related distortion in the pass band
less load on the driver above tuning, allowing it to actually move some air in your room.
We pay for long xmax, why not actually use it in a productive way?
I think the 6" id port is small, and to top it off the higher you tune the larger the port needs to be to keep vent velocity/compression/noise acceptably low.
I suggest you pick a maximum port length based on what you consider the allowable lower limit for port resonance, a 6" x 27" port resonates at ~208Hz, I've been informed 200Hz is the cutoff
Port size vs box volume:
6" x 27" it takes 584 Liters to get an 11Hz tune, switch to 8" port and the tuning goes up to 14.25Hz. It takes a volume of 980 Liters to get the tuning back down to 11Hz with the 8" x 27" port, it's all realtive.
And to top it all off the 8" x 27" port has a resonance of ~195Hz, that some will tell you is no good.
Pick your comprimise, or go big