"The key to the question of "what happens if we take out Sadr?" is how Sistani would react."
There are an infinite number of possible unintended consequences to anything we do.
Whereas the condition called "winning" is finite and measurable.
If taking out Sadr brought Sistani off the sidelines and into the anti-American Shiite camp, it would not do anything to help achieve the "winning condition" you reference and would likely hinder it immeasurably. We presently have a large majority of the Shiites not actively seeking to undermine our efforts. Want to change that?