If I remember correctly, the "pressure plate" in the S8 cartridge actually isn't a real pressure plate, since it has a predefined distance to the gate -- even without the film.
I am not 100% sure about the specs, but I think the defined distance is between 0.013 and 0.015 mm. Kodachrome was about 0.014 mm, so it basically moved without any "pressure". The plate in the cartridge is more guiding than building up pressure.
If you refer to Avichrome regarding Polyester film, it is about 0,0135 (from the top of my head as well) so does not need any tweaks. I tried it in about a dozen of cameras and none of them had any issues with sharpness jitter (nor bad registration).
Polyester in my opinion is more suitable for the S8 cartridge than any TAC-base, since it does not remember bending. the torques needed to accelerate film that sit in the cartridge for a couple of days is much lower than the torque needed with TAC-based stocks. That actually improves registration significantly.
Or what are you planning? :)