Guessing subs in 300 BO aren't going to require anything over what 9mm would need, but that may be wrong.
There's a bit more exit pressure and heat input, so stainless or Ti baffles would be a good idea, especially with the accompanying higher rate of fire typical of AR use. SS or Ti baffles can be made thinner, though; done right, there's only a 2-3 ounce penalty with steel and it can be a wash with Ti.
ECCO have you experimented with features around the bore to create more turbulence inside the cones you plan to invert? Wondered if that might help the success rate.
I haven't. I'm happy with 114 dB from a 1x5.5" can.