How long do we need to wait, until the DX11 client?
More or less.... yes. Soooo many of this games performance/stability issues revolve around the shoddy DX9 implementation... and Microsoft's lack luster support for DX9 (and by extension the vendors as well with their drivers in the last several generations). It's nothing new... and it isn't isolated to just XIV either. There are many DX9 games that have issues with the newer generations of cards when they are running in the newer versions of Windows.

Have to remember that DX9 is now run through an emulation layer. It hasn't been natively supported by Windows since XP. That was the last version of Windows where DX9 was supported properly by Micro$oft. It wasn't long after Win7 came out that we started seeing issues with DX9 in the video card drivers as well. It is more or less because of the fundamental changes in how Windows handles graphics, and that has also extended into how the drivers are coded as well.

So, yes... the DX11 client should resolve a lot of problems for people with cards released to market after around 2009/2010. I just hope they do a better job with it's implementation than they did with the DX9 client.