First off, when you dial up 14 volts on the DCS remote for a variable track, it's not necessarily really 14 volts. The DCS remote assumes 22 volts input and divides it's speed steps up with that assumption. It changes voltage in 1/2 volt steps, so there are 44 steps. So, that 14 volts would equate to step 28. Now, let's assume you are putting 17.5 volts into the TIU instead of their assumed 22 volts. That means you're only putting 80% of the voltage that the DCS is assuming for track voltage. The indicated 14 volts on the DCS remote is really putting 11.14 volts on the track.
Next, consider the vagaries of the chopped waveform generated by the TIU in the mix, and it's difficult to know what amount of energy you're really bringing to the table using the TIU to regulate the voltage.