The secret is that they are not actually *extenders* - they are, as you said, *deflectors*. They are most useful to direct the flow of air where you need it... and if you are tall enough that you can't completely hide behind your high windshield, then the extra bit is not to *raise* the airflow but to *redirect* it directly into your helmet. Because flowing air is much quieter and easier on your neck than turbulent air!

So... Use your hand while riding to find the bottom edge of the turbulent windflow coming off the top of your windshield. Just move your hand up and down, it will be obvious where that border is. If it's below the tip of your helmet, then you want the extender pointed "down", i.e. *the top edge of the extender should be pointing directly at your visor*. This will create a flow of smooth air towards your helmet, while the windshield keeps the wind pressure off of your chest.

Entirely depends on how much money you spend on it. I had an MRSA X-Screen that worked wonderfully, even at high speeds. I also had an aliexpress one that looked identical, but was crap - wouldn't stay in place, would vibrate to hell.

Some require you to drill through the screen. The MRSA X-Screen doesn't - it has friction options where you use screws to push pieces of rubber against the screen - and at worst, that will leave some marks you can buff out.

Can't imagine they ever would be.

In my experience, yes.