I have a hard time justifying expensive character models, especially when attached to smaller units, and even more especially if they'll just be sitting in a transport most of the game. Also we need to take a look at the list that one of these weapon batteries would fit into.
It sounds like you take multiple Night Scythes which lead me to believe you are probably fine for anti-air weaponry. Does the rest of your army plan on being right in the face of the enemy? If so maybe a poor ballistic skill Battlecannon would do more harm then good.
Perhaps your best option could be replacing the small unit of Immortals and this Lord with another Harbinger of the Storm (love them) and a larger unit of Warriors? More shots, more possibilities for glancing hits. Vehicles should be popular once again and Necrons love to laugh in their face. This combination also works well against infantry because of the number of shots still fired.