Next time, do not be so quick to question Bob "The Knowledge" Khan, for if you do, you should prepare for a schooling.
I have a 4890 and DX10 doesn't cause any graphical issues but it does take maps about 20 seconds longer to load than DX9. I have a high mouse sensitivity so didn't notice mouse input lag.
Eulogy (Hippos) also has a 4870X2 and he has the same issue with DX10 loadings against DX9. It seems that ATI do have a few issues to iron out to say the least.
I would definitely stick with DX9 as you probably won't notice much difference visually and it runs a bit better in my experience, especially for ATI users.