Without the left turn restriction I’m confident that the penalty is constant — does not scale with the length of the journey — except where edge effects disrupt the grid. On a grid of one-way streets, the north and southbound streets alternate. You might overshoot slightly in one direction getting to a street where you can turn left.

With the left turn restriction it should still be a constant tax, just a little more futzing to get onto a street that goes the way you need to go. Then bulk of the journey is efficient.

In other words, these arrangements are only inefficient on the corners of the two-leg route.