Does this increase performance? I used to have a lot of frame stuttering when playing PSOBB. So far seems to be the same little bit of stuttering. I don't see much of a improvement.
D3D9 has a definite performance increase over D3D8 - without changing anything else, it made me stop dropping frames around Merissas, in certain annoying parts of Desert, etc. I've heard some reports of D3D11/12 further improving frametimes and stuttering over D3D9, but it didn't do anything noticeable for me.
Also, with D3D9 or above, you can use all of the new fancy antialiasing options.