okay... britbrat, as you mentioned before and minnflyer reiterated, regardless who's right, it won't make a noticeable difference in terms of speed.

as for extreme planes, you don't see them with dihedral or symmetrical wings because they are designed for speed alone. thus, it is down to less drag and more horsepower. therefore they have SHORTER, MORE AERODYNAMIC WINGS (to reduce drag) that are asymetric with no dihedral (so they are as efficient as possible give their span and thickness) so they create just enough lift to maintain level flight at their intended racing speed.

anyway, if you want speed, clip the wings and you'll get it. all this dihedral/no dihedral, nothing you are saying is wrong. now THAT is a statement you cannot argue :-)