I have a TC Sounds LMS-R Ultra 5400 18" subwoofer in a 7.3ft3 box after all displacements. It is tuned at 20hz. My port is 16" wide x 3" high. So 48 square inches. I don't get any chuffing even with test tones. Would my sub sound better or give more output if the port were substantially bigger? I know when you listen to an SVS PB16 Ultra and 2 of the ports are plugged the one port is still enough for it obviously so why would a bigger port help me?
How long is the port? The port should be 31" long for that size opening in that box. If those are the dimensions you have then the vent air velocity is 22 m/sec, which is correct and will not chuff.
That gives you an F3 of 27 Hz. That driver does not go particularly deep. It is a high spl sub.
The optimal cox for tightest base is 3.5 cu.ft. Correctly ported your box is an extended bass alignment, which lowers F3 from 34 Hz optimal box to 27 Hz with your alignment. So not a lot of extension.
If you made the port bigger then you would get into port resonance issues to no purpose as there is no point on lowering the vent air velocity further.
I hope this is a properly tunneled port as if that is just an opening, then you have a real mess on your hands, that can not even be calculated.