MY POINT IS, she's proven that she isn't that weak. Susano'o is basically the ultimate defence( RibCage or otherwise) but her toxic gas was able to be able to melt susano'o's ribs and corrode sasuke at the same time. That coupled with her lava release to trap people (which will kill you if it hits you) makes her formidable. Even madara said the lava release shouldn't hit him, and it even melted his susano'o ribs also.
If mei Releases her toxic gas in the area she can potentially kill large armies at the time, it's seen that she's immune to it. Mei isn't that weak. For all you know she could beat A - Encase herself in a lava shield and concentrate the area with her toxic gas. IMO, Mei puts up a good fight, if not beats the current kages. She really isn't that weak....
Like i said, 4th kazekage is far weaker. His gold dust is basically only good for economical purposes - he's a watered down version of gaara.
Tl;Dr; Mei has melted both sasuke's and madara's susano'o