Reasons for needing the 'u-bend' that spring to my mind would be
1.Simple packaging constraints caused be the unusual shaped sidepods themselves, the manifold itself could impede a direct path to this area.
2.More exhaust length is needed to convert the round pipe at the manifolds to the slot-shape at the exit, a sharp change in the exhaust pipe could limit airflow?
3.It prepares the air somehow, my knowledge of fluid dynamics is little to none, but could the u-bend even be a spiral to 'slingshot' the air to either increase speed or shape the airflow?
Slinging mud here and seeing what sticks
Re: Red bull, I think the concept for what Mclaren are doing with the air here is very different, RedBull are taking a direct path the blow the outside edges of the diffuser, whereas Mclaren seems to be trying to use the exhuast to draw more air in and over the top edge of the difuser and away from the rear tyre.