There is a linear (relativly) portion then there is a non linear portion, then saturation.
If you are within the linear range the most voltage will be produced.

I did some work transformers for my project at uni, been some time so i cannot remember everything, but can remeber that saturation caused some very weird effects.

As i said omex found this out through actual experimentation so i`m not spouting theory here (i was beside them at the detling kit car show).

When saturated more energy will be released just over a much longer time hence the lower peak volatge.

Darren