"... achievable ... ?"
Absolutely, since unloading doesn't happen all at once. Your driver doesn't just reach the edge of a cliff an fall off. At the tuning frequency, whatever the actual value is, there will be a useable range on both sides of the "cutoff".
One beneficial effect of using the horn shape for a port is that the actual cutoff rate (in dB/octave) is less steep, meaning it will resonate across a wider range, instead of just a narrow band centered around the actual tuning freq. Another benefit which we are all interested in, is that whatever energy reaches the horn, after loading the chamber, will be better coupled to the environment (amplified).
When considering reflex ports of all kinds, try not to think in terms of a few frequencies, but rather in octaves. Depending on (many) design factors, some ports roll off at slightly more than 6dB/octave, which is the natural driver rolloff in an infinite baffle, while others rolloff much faster, even 24dB/octave because the driver output and the port output are approaching 180 degrees out of phase with each other. In that case you will have a port which, instead of suppressing excursions, actually helps to cause extreme excursions.