35 mph WOT power loss

You could even tie it all the way back with a zip-tie, and that will cause the shift to NOT occur until way up the Rpm band, so you could at least know if you are in the ballpark.
I think what Rusty is alluding to, is that the length of the lever on the trans,which determines it's "ratio" has to be synchronized to where on the throttle arm, the other end of the cable is working from.

If the trans lever relative to the throttle arm, is too long or too short, then it will not downshift satisfactorily; being either too late or too early.
Furthermore the normal shift quality could be too soft or too harsh. And as you may be discovering, there is no happy medium.
Furthermore, it is possible that with the cable pulling the lever all the way back, that the secondaries cannot fully open, or conversely, that WOT cannot pull the lever all the way to the back. But I think you might have that part figured out.