Not everything you mentioned is caused by the gfx driver; some of those problems are simply wine-bugs. fglrx does not support DirectX, neither do nvidia's drivers. That's handled by wine.
Wine uses different code depending on the driver and it's advertised capabilities, it's quite possible that you're running into different code-paths that aren't well tested or simply buggy.
Figuring out which part is responsible for a certain bug isn't easy, though.
But yeah, wine + fglrx isn't the best combination. Wine's DirectX layer was initially developed on nvidia-hardware, it's not really optimized for fglrx. It's making progress, but not as much as you might hope.
You might improve performance with some wine settings (the defaults are often targeted to nvidia), there may be tips here:
In any case, statements about a card's performance that are solely based upon WoW on wine are a bit lopsided. The 5750's hardware is a bit faster than the 9800GT, but not much - you're comparing a mid-level card of the current generation with a high-end card of the last. It's slower in your use case due to driver differences.