You aren't so much being outmanuvered...you can still fleet of foot, and you have super fast anti-grav technology: you won't be out manuvered by anything short of Eldar. What's tricky though, is SM can spread their forces nicely across the table, because at worse they have a 3+ save protecting them more than half the time. What's key, and the true definition out out-manuvering, is to allow your opponent to think he has you boxed in, and just before the jaws close, you end up behind him. You can fly over enemy units with skimmers, so yes, I do mean that quite literally. Having a raider full of wyches with a distinigrator can devastate whatever squad you want, counter attack or not.

Deception, takes a huge roll in real world warfare and in Dark Eldar fluff. Adding elements of it to your gameplay can work wonders. I'm not talking cheating of course, I'm saying stacking up so heavy on one side (providing key units cover) which can lull your opponent into a false sense of security, and cause him to have to come to you, which due to our range, isn't always out of our best interest. What you lack in armour, you make up for in speed. Take full advantage, and make him play your game.