"the measurement also looks to be swamped by the modal distribution of the room the speakers are in."
It looks like there is a room mode (null) at 40hz.
"which seems to be showing that the port is essentially doing nothing compared to simply making the rear volume larger..."
Simply making the box larger gains at 31hz, but you lose 3dB at 50, 63, and 80hz. The vented (without EQ) has about 3dB more output than stock at those same frequencies. While the vented has only 2dB more output than large sealed at 31hz, it's 7dB more than stock. When EQ is applied things change. EQ applied at 31hz will cause distortion and increased cone motion on the sealed system, on the vented system the EQ is at Fb and thus does not eat up cone motion or increase distortion. It also protects the system from excessive infra-sonic inputs. The Q=2 at around 31hz will bring up 31hz by about 6dB, and the 50hz by about 3dB, little change above there. The response with EQ fits in a 6dB window (as measured in the example here) vs 19dB for stock (ignoring the 40hz null).
Fletcher/Munson contours of equal loudness are 10dB apart midband, but only 3dB apart at 31hz in the 90dB region (where most people listen). That means the un-EQ'd response difference at 31hz is going to sound much, much louder.
The original concept was based on the computer model for the Eminence 15711 (a custom-made driver based on the K-43) as a 6th order equalized system. The lighter magnet K33 is a reasonable 'fit' with some ripple in its response (±1dB or so in the model), but sounds a bit 'heavier' than the K43. The concensus is that the 7" ports sound better, they model within 1dB. When measurements are that close I always go with what sounds better, all but one pair I have supervised are now 7" (the owner of the lone 10" pair likes the 'heavier' sound on his RAP music).