What you are talking about is "ram air". And it works, but only
slightly. The problem is the air drag created by a large funnel will
negate any increase in power. Also, to use that much air, you have to
use more fuel. But ram air is for real and most planes intake their air
from the front, right behind the prop, right where it is optimum. You
get a slight increase in air pressure from such an arrangement. But you
can't do it if you increase drag as well. A very large funnel (larger
than the nose of the aircraft) would create so much drag, the plane
would not go as fast. It would make more power, and consume more fuel,
but the power created could not overcome the drag created.