Not sure about models without the Bose sound system, but the Bose system in my 2SS is insane!
With the top up, I can hear some wind noise at highway speeds (speed limit by me is 70 mph), but the speakers easily overcome that without having to turn the volume all the way up.
When the top is down, the speakers get an added boost - don't know how else to describe it, but when the radio is on and I'm putting the top down, as soon as the tonneau cover is in back place over the stored soft top, there's an audible shift in output and the volume is enhanced (deeper bass, clearer highs)...perhaps it's just a jump in volume to compensate for the additional road/wind noise, but whatever it is (and I apologize for not having researched this before responding), to me it's glorious.
The Bose system can get ridiculously loud without any distortion (with top up or down) - I haven't been bold enough to crank it all the way up, but at about 3/4 up it sounds (and feels) like a high-end aftermarket system. Any louder and I think I'll give myself tinnitus.
Hope this info helps with your decision-making process!
(Edit: I realize that I did not acknowledge or answer your specific question about sound pressure readings in coupe vs convertible. I have not.)