Finding a right aftermarket screen is a nightmare. Wouldn't it be ace if you could try before you buy? Lol
When I had my 750GS, I had similar issue to you in that I also tried a few different screens. For me personally, the Powerbronze Touring screen caused bad buffeting, the Wunderlich Marathon screen was better, but the best I tried was the GIVI screen which was spot on. Obviously the screens affect different size people in different ways. I'm 6'1" to give you an idea.
Since I got my 850GSA I've find the standard adjustable screen to be good, helped by the two additional smaller screen deflectors fitted as standard above the indicators, which push air around you.
I have added one of those low cost universal extender wind deflector screens off FleaBay (see pic below) that allow you to adjust the wind direction (and therefore reduce or increase airflow) on the move. It works great and is a cheap solution.
I can now tour for many miles at motorway speeds with virtually zero buffeting. And if it's hot, I can simply lower the GSA screen and tilt the extender towards me so I get cool air flow.
Not sure how much helmets make a difference, but for reference I use a System 7 Carbon from BMW.