I'm obviously all for the idea
In that first picture, it's comparing max output and extension without exceeding excursion limits above tuning between this design and a traditional dual RLp-15 LLT. In the picture I am attaching, it's the proposed design vs Rodny's current IB, both at max output before exceeding excursion limits at 11hz.
As you can see, BIG difference. I've been suggesting adjacent room LLTs like this to those wanting to build IBs for a while now, but most have been scared to try it - I can only guess because it's not common....hasn't been done yet as far as I know. What people don't realize is that there is always room for improvement with a port. No matter how low the natural response of an IB goes in room, if you port it and tune it to the -6db point, you gain more clean low end as well as more usable headroom throughout the sub's range while maintaining a flat FR. You also avoid having to use lots of EQ to boost the low end.
I design LLTs with no port resurgence to better match with room gain, but in this case, the tuning is so low that I felt a 1.5db resurgence would actually be good to help counter some of the effects of electronics rolloff. I can't wait to see how it turns out.