Whether you're using two bricks and a TIU or a regular transformer to supply power to the track, it shouldn't make a difference how the voltage gets there - after all, 14v at the track is 14v.
There may be differences attributable to internal voltage loss in each system and degree of accuracy, different wave forms and insuring the TIU is getting a full 18v to start with, but once those differences are accounted for and each system adjusted to insure 14v to the track, it shouldn't make a difference to your engine whether the 14v is coming from one or the other.
Assuming you have the Lionel transformer to test with, I would first dial up 14v to the track through the TIU and measure the track voltage with no load (no train on track) and then with a load and then repeat using the transformer and compare the results.
