I just played with it some more and part of the problem seems to be that MAME attempts to get exclusive access to the device. From what I can tell, that means any other program that needs exclusive access, which you need for FFB, will need to un-acquire, acquire exclusively again, and re-download the effect anytime it wants to make start or change an effect on the device. That three step process created stuttering; at least for me. If I change src\osd\windows\input.c so that MAME acquires non-exclusive access this problem goes away, you don't need to reacquire, and things work well. Except for one thing; MAME start complaining that it is 'Unable to create the Direct3D device (88760868)'. Everything seemed to work fine ... it just complained.
Maybe this is all piss in the wind, you know this, and have a solution.