This is irrelevant but given that I was mentioned and kind of misunderstood, I thought I'd respond. I didn't I said you would get higher FPS all the time with DX9 vs DX8. The more efficient renderer is rendering more, so it's just doing more, faster, but in more time than something that's doing less, slower. So it's understandable that it's getting less average FPS.
Anyways, DX9 can help a lot with drops, latency, resource usage, frame timing and performance over longer periods of time even if it isn't getting you higher average FPS. Finally, keep in mind that how you test dxlevel can affect your benchmarks. You should be setting dx9 and then launching without -dxlevel 90, or else you will experience some graphics options getting reset, causing lower FPS.