I know it's hard to put these polls together and hard to predict all the possible answers.
Almost all my hives have SBB. I have a lot of slatted racks and I use them with SBB. The slats all run the other way (opposite the frames) on all of mine. Since I'm moving to eight frame boxes and long hives, and since they aren't available for eight frame hives, I probably won't buy more slatted racks. Also, I think the ventilation provided by the slatted rack can be, at least partially provided by the SBB.
Also, I had one hive that FILLED the slatted rack space underneath with comb. What a mess.
I leave the "tray" in most of the year and just pull them out during the heat of the summer for more ventilation. I don't think leaving them out over winter would be a good idea with the wind and the cold here. I get 60 mph winds sometimes (100 kph) and -20 F (-29 C) sometimes in the winter with a week or two of -10 F (-20 C) pretty normal.
Another nice thing about SBB is the water runs out. You don't find a puddle of water on the board because the hive settled a bit and is now level or slightly leaning back. You can level the hives and not worry about accumulated water on the bottom board.
I find a small cluster in the spring trying to raise brood does better with less cold to deal with. But a booming hive in the summer makes more honey with more ventilation.
As far as the Varroa issues, I find the SBB is more useful for monitoring than for controling Varroa. In my experience, a SBB will not make a significant difference in the mites, with or without a slatted rack.