As I stated in another thread,

http://www.rcuniverse.com/forum/m_3194805/tm.htm

I think that diehdral wings are more draggy than the same wings with no dihedral.
Please read my posts in that thread on give your opinion.
As a conclusion, I would say that a speed record plane shall have no dihedral.

Technically yes, because it results in less lift from the same wing, & it very slightly increases induced drag (unless the dihedral is extreme) --- but I doubt if you can notice (or measure) the difference.

Actually, I used to put a touch of anahedral on my high winged pylon ships. That way the wing flatten out in the turns.

If you ever watched a pigeon glide, the effect has to be really minor. Those tree-rats use about 45 degrees per wing, and cruise pretty flat. Don't ask me how, I haven't a clue.

I have the answer. Pigeons feature an amazingly complex feature. I mean, they not only have variable wing span, but their wings can just morph during flight (till now no human aircraft manufaturer has succeeded in doing such a fine mechanism). During gliding, true, they use some kind of really high dihedral configuration (just a lazy way to keep a constant heading), but they tend to alter this dihedral during high speed flight.

Thank you GOD for creating such beautiful aircraft !!!

The Wright Flyer used a morphing wing to turn, and nasa has also been testing some aircraft with the same concept. Of course, no human made wing can be morphed like a birds, but there HAS been morhing wings in airplanes.