Meggan can probably fly at, at least, Mach 1. She can match Captain Britain with relative ease, and he flies at Mach 2. She should logically have reflexes to match and thus would get the first strike.
I'm going to go with no, considering when she became part of the Braddock waterfall she was basically just living water, and didn't technically have a brain per se. May depend on what element, e.g. if she assumed an air form it may affect her. Iunno.
Meggan is no Storm. She may be able to control earth, air, fire, water, but she does not wield any of those elements on the scale or with a force that approaches Storm whatsoever. She is a jack of more than one trade, but a master of none when it comes to elemental power and control. Her metamorphic abilities, blasts and strength are her strongest suits in this fight as she does not wield the elements with enough power to beat Sue. Sue beats super strength and energy blasters.
__________________ Ann Nocenti writes on Storm: "At her command oceans rise, tornadoes destroy, rains pummel, breezes cool, sun dries, seas part!"