Normal powerheads squirt out a thin, high velocity jet of water, the seio's and tunzes pump out a wider, lower velocity jet of water, and take it from me there's a difference in spread and how effective the flow feels when you're a few feet away.
One of the typical surprises people get when they retrofit tunzes is that all of a sudden a ton of debris gets kicked off the liverock, and placed into suspension. This soon clears out, and is a good thing, as debris settling on your liverock is very, very bad for algae formation.