While the sub can go into the midrange, I'll be cutting off at about 300Hz and let the actual mid driver take care of the rest.
I think that would be low enough.
I guess that brings up a good question: are isobaric speakers uni or bidirectional in their "use" of sound?
The radiation from the front of the outer woofer's cone would have the same pattern whether the enclosure is isobaric or otherwise.
Not too hard if I can help it. The main thing is it's about 10db shy on sensitivity compared to the mid/hi, but I'm only probably going to be running those at either 10 or 20 watts at most.
To a first approximation, you'll need about ten times as much power going into the woofer section in order for it to "keep up with" a midrange/tweeter section that is 10 dB more efficient. And note that a voltage sensitivity comparison does not translate directly into an efficiency comparison if the drivers have significantly different impedances.
I thought one of the main selling points of isobaric is it didn't need as much space?
That's usually how it gets "marketed". Ime there is a worthwhile improvement in bass quality resulting from using a larger airspace behind the inner woofer. This also increases the susceptibility of the outer woofer to over-excursion, which is an additional trade-off. That being said, imo unless minimizing enclosure size and/or maximizing excursion-limited SPL is the top priority, you might consider sizing the airspace behind the inner woofer more like you would if the outer woofer was not even there.
If this is to be a vented isobaric, I suggest including some provision for adjusting the port length, as I'm not confident that normal vented box modelling will be sufficiently accurate.