Well, as you said, the sound is not as full or open as in a modern system, to say the least !What strikes me is a terrible loss of treble !.
But I suspect the issue is more related to speakers location,
recessed in the footwells, than to speakers specs (power rating, impedance..)
In modern cars, speakers take place most of the time in the bottom of doors (woofers), while the tweeters are generally located in the upper part of the doors, or in the dashboard itself (which makes sense from an acoustic point of view!)
I'm still doubtful if changing my "modern" speakers for classic Becker's would improve things... should a vintage car have a vintage sound, that is the question