I don't know who it would fall on, but if an XP shower is set on a tank with a large amount of XP (50+ levels) and let drain with no one to accept it, the resulting flood of individual xp sprites clogs up the frame buffer and brings framerate down dramatically.

So maybe don't drain if there is a lot of XP under/over the drain? That seems like a good solution.

"Bug: If I leave my water running, it floods my home. Plz fix" :P

I see no point in adding more logic to showers. It would be like fixing dispensers to prevent user from dropping entire AppEng net contents.

There are multiple in-game solutions to this (redstone control, drain, etc)

Arguably, floods don't destroy your framerate - that's the problem I had with it. If it's not worth putting logic on, then it's not worth it.