Yes, the TPS will be related to the pressure inside the trans.
As engine load goes up, the PCM commands higher trans (line) pressure.
The engine load is calculated from the TPS, MAP and MAF. I don't know the exact formula, but the TPS is important.
It is critical that those sensors work properly to prevent both trans and engine damage.
The builder may have "fudged" the EPC (pressure control solenoid) to increase line pressure; it is kinda the poor-man's tune, but he may have done it to save himself another rebuild.