Interesting idea with the closed cell foam, though it might affect heat dissipation in some components (admittedly the current is low so there isn't too much concern as long as the components were chosen correctly). I think it'd be easier to just find a way to seal the cone with a gasket or O-ring.
I'm not the best person to comment on the sound of the Takstar, I don't have that kind of experience and my goals are for nature recordings instead of voice or music. And I'll be honest I haven't even plugged them in before I did the teardowns. For the price they're probably going to be the best you can get but that's mostly based on the construction quality and from what I've heard from others - there just aren't a lot of data on these low cost SDCs.
As far as hypercardoids, I haven't seen a lot of them in this form factor, your best bet is probably a shotgun mic, or you can get the Takstar CM63 and the 3U Audio hypercardoid capsule on it.
-Steph
