Why aren't vent stacks usually equipped with hats?
You are right about the Bernoulli effect.  Additionally, I have had customers who have complained about odors in the front yard when the wind is just right.  

As far as emptying the toilet bowl, the government may need to bear this responsibility.  Manufacturers, in their rush to meet the demands and constraints of an out of control national legislature, requiring ultra low flush commodes, weren't able to get all the bugs out in time.  They had to meet the requirement and still keep product on the shelves.  Some of the early models had this problem.  I think most of the later models have this problem under control. It has to do with how they flush the bowl and how they keep the bowel filled.

As to a fix, here is an idea that may work. It is an engineered concept and works in some situations.  Say your problem stack is a 3" ABS riser sticking through the roof.  Take a piece 5" diameter of the same material about 8" long.  Fix it to the top of this stack.  This sleeve will run parallel to the stack. Put equal sized spacers between the stack and the new sleeve to drive your support screws through.  The sleeve should not touch the stack.  Extend the sleeve above the top of the stack about 4". The sleeve will also hang below the top of the stack 4".  This sleeve will break-up the wind currents and take the pressure off the top of the stack. Good Luck.