Cool, dense air holds more oxygen than warm air. One of the best mods I did to my supercharged motor was adapt in a cold air intake. Dont take in combustion chamber air from under the hood if at all possible.
As for stock intakes, any adaptations should remove corners. Corners restrict flow. Diameter matters, but unless you are increasing the combustion chamber size, you really arent getting substantial increased flow, and stock size will probably do just fine.... unless there was a flaw in the original design for engineering or production purposes.

Oh...you can also use these cold air intakes to produce a little ram-air effect as you are driving along at 65 MPH...effectively giving you a couple lbs of positive boost on the intake side.