They add 10 hp each due to the wind turbulence funneling around the plane very much like a supercharger in a car. Add more you obtain more power. Self-appointed expert in airplane scientology.
They are stick on Foo Foo Rah... a device physiologically engineered to worry your aircraft competitors that they aren't keeping up with the latest technology...
Through the equivalent of Harley-Davidsons with straight pipes, they're there to make whistling noises to add to the noise the aircraft makes so that you aggravate the neighbors more when you do a low fly by.