I believe he said top case which is behind him when riding, not a tank bag.
I know it might be a matter of wording, but I will bet it isn't that the turbulance comes from under the tank, but that the stream of air coming from under the tank interacts with air coming from the air above and besides and under the windshield and from air reciruclating behind the windshield and this causes the turbulance. Little changes in attitude of the bike, wind, etc can make total changes in the setup that works. It might ibe in this case the topcase weight affects the attitude of the nost a bit, or that it changes the flow of air that is recirculating around him and that changes the interaction.
It likely won't be any one thing that fixes it for everyone.
I'm 6'2" tall..When I added a Corbin seat to my 650 I had to change everything around. I have the Madstad with the original windshield attached in the suggested (middle) position to the bike. For me pulling it all the way up and back worked best. With the Corbin (and the extra inch or so higher I sit) I found for me the windshield in the lowest adjusment allowed by the madstad worked best.
(Me new Givi + Madstad seems the best, though.)
Your findings somehow confirm what more and more strom owners are figuring out: the turbulence comes from under the fairing, not from its top. Adding a top case could somehow alter the general aerodynamics in a way that makes it draw more air from under the fairing, therefore, increase turbulence.