Arche: Well, I can only judge from my experience, and I only got that effect (like, 'half-assed motion-blur'?) to work with DX6.1. Using a Voodoo 2. TEH KING!!!!1
Either way, the motion-blur unconditionally shoves a big stick in the cogs in my system. I'd say going through downgrading DirectX is a lot more pain than it's worth, though. Sure, a part of the final battle will hardly look at all, and when It's Done the problem strikes back. Eden and Invincible Moon are also affected, but I wouldn't call that a very big deal...