For whatever it's worth, my 85 300D is missing the resonator. No adverse effects or high noise levels that I can tell. Last fall the section that descends from the header failed, and lacking proper replacements, was temporarily fixed with a sleeve. I've been wondering whether to go back to the resonator configuration, or leave the system with the single rear muffler.

I also have an 85 300TD that has the resonator: seems actually a little noiser, though currently it seems peppier than the 300D.