Eric, I am not all that well versed on T/S lenses and my comments will probably be of little help, However I have had some decent success using a FD 35mm Tilt shift. I use a Rainbow imaging adapter adapter.
I also have a similar EOS adapter & have great luck using it with my 400/5.6 lens & the 300/4.(always wide open, I do not use the adapter Iris to control light) I cannot imagine anything other than the adapter being the problem. There does not seem to be any reason your EOS T/S lenses should not work like any other adapted lenses.
Most likely not related to your performance issue, but I am curious. I assume you are presetting the aperture with an EOS Camera. Since the FD lens is a manual aperture lens, I can focus wide open & then stop down to meter & expose. How do you manage with the EOS lens. I know it is manual focus, but isn't the aperture controlled by the camera control wheel? seems to me it would be hard to focus stopped down.
I do know that once you tilt or shift the FD lens the distance scale becomes unusable, it even mentions that in the instruction manual I have for it. I assume it would due no different with the EOS lens.
I hope you find a solution.