Being that you're familiar with studios, you are surely aware that no car can ever be like a true studio setting. The odd shapes, different materials, and different acoustical attributes of a car cabins are just not conducive to a studio-like experience.
Have you looked at dynamatting your vehicle, to address some of your resonance issues?
I'm no audiophile, though I have spent some time in a studio, I just don't have the money or the time to be dedicating to high-end audio in multiple vehicles. But I am the kind of guy who will take SQ over SP any day.