Every idea has some merrit, but I think that you will get a lot of interesting responses from not so polite people. I will take a stab at giving a few reasons why I think it wouldn't work. You can compare what I say to other people and then decide what part of it is knowledge to take away with you.
In general, 'rifling' implies spiral grooves through the manifold passages. There are so many factors based on size and spacing, but I think they would only create more drag as they provide more surface area in the same pipe. The closest thing I can think of to what you are talking about is General Motors' Vortech V6 manifolds. They use a proprietary manifold to increase the velocity of the air going into the cylinders that involves a circular path of travel. I would go and research that if I were you and learn what they did.
Some other people may have some other ideas, but in general, the straighter the path for the air to travel, the less it loses its velocity and the less drag.
warmonger
