The overboost protection solenoid gets a signal from the pressure switch on the manifold above the IP. That circuit has nothing to do with the transmission.

The vacuum amplifier is more complex. I haven't figured it out but I think the only wires are to a coolant temp sensor.

If it shifts hard because it shifts late, you'll have to figure out the vacuum amplifier system if it's not just a bad line to the modulator. If it shifts hard but at the expected rpm then you can adjust the modulator to soften the shifts. If you set the modulator so shifts are imperceptible like in a Lexus or Chrysler you will drastically reduce the useful life of your transmission. The 'normal' shift feel of these transmissions is more akin to a 90s Honda AT.

Sixto
87 300SDL