What's the headroom in the aft cabin between the bunk surface and the
ceiling?

I don't know exactly, but with my wife on top there's enough room for
spirited fun. I'd say it's perfect. There's also standing room just as
you step inside. It's not as nice as the aft cabin on a Catalina 34 for
room, but on a boat that really performs, it's unexpectedly nice.
There's also a single opening port and 12V vent keyed into the second
zone air conditioning and heat. We'll probably just leave the door open
on most nights.

RB
35s5
NY