I feel like your "snake" passages are too small. Assuming everthing is at a constant height, your fan is drawing through a duct 1/4 its cross-sectional area (and due to boundary effects, probably only able to pass 1/5-1/6 the volume of air it can). Obviously you have to work within the restrictions of your existing engine compartment, but something like this would be much better (keeps ducting as constant area as possible)... sorry for the potato-drawn image.
