The front speaker is kind of sealed, but not like in a regular sealed cab. In a regular cab, the moving speaker causes the internal air pressure to go up and down, which in turn dampens the motion of the speaker. In an isobaric cab you don't get this as the speaker behind it moves foward in unison, keeping the air volume and pressure between the two speakers almost constant.
This might be a negligible effect though so for all intents and purposes it could behave like a 'best of both worlds' cab as you suggest. All I know is they sound great, deep deep lows, punchy and vintage sounding all at once (and I only have the SP210).
Thank you! that's the answer I was looking for.