My competition car scored 116db in the SPL tests which was on the low side since I was tuned for sound quality. The average was about 118-119 with the loudest cars hitting 125. I'm running the same power in my Boxster and from general impressions, I'm probably hitting very close to the same levels.
With the top down and the windows up, the Boxster cabin is an amazingly quiet and refined place to be, and you can enjoy your tunes at moderate listening levels, and still be able to have a conversation with your passenger or be able to hear sirens. On a busy freeway, the noise from other vehicles becomes a problem and you really need to crank it to overcome the noise, so it's nice to have that flexibility.
Also, the decibel scale is logarithmic, so every 10db gain has 10X the sound intensity, so a 30db gain actually has 1000 times more sound intensity which qualifies as hi-fi in my book.
While I don't think it's ever a good idea for a driver to wear headphones, I'm also a headphone junkie and got my start in real hi-fi with a pair of Sennheiser HD430 headphones which were an amazing headphone in it's day and still one of my favorites to this day. Headphones are great to use as a reference sound due to their very low distortion levels and high quality per dollar spent. A very good reference headphone to use to setup car audio systems is the
AudioTechnica ATH-M50x. They are very clean with strong, extended bass that is slightly on the heavy side, but is what you want in a car environment. If you can setup a car audio system to sound like these, you have done very well.