Something tells me that dealing ~2500 damage in total to your entire team with the press of a button is a bit ridiculous. Not to mention it can be extremely disruptive to your other team mates, especially health stacking Tanks (seriously 20% of their health is almost 1000 HP).
The Wall isn't in anyway spam-able. It's got a 26 second cooldown for the majority of the game. If you REALLY want to give it a resource, make it use his Flow.
Lower the base duration to something like 2 seconds. Depending on what percent of Flow he has when he casts W, it increases the duration for up to 2 seconds. Using W also consumes all of his Flow. This way, Yasuo chooses between getting his Shield or using Windwall.
I personally don't think any of this is necessary, but there's no way that it should use 20% of his Max Health AND surrounding allies to cast. It's insane. Where did you even get the idea of stealing ally health? Heck, with that effect it may be more efficient to just not use it, since it's likely to deal more damage to your team then the enemy attacks that it negated would.