Got there before me. As a quick sanity check, it might be worth firing up a hex editor and scouring the EXE for each variable name before you delete it. We know PST has a lot of hardcoded shit, and I imagine embedding variable dependencies in the binary is not beyond it.
Does using an EXE patch rather than an effect to adjust the speed allow the creature to respond correctly to haste, slow, etc.? If so, could a similar patch be applied to player characters to recreate the BG1 walking speed when playing BGT or Tutu?