I teach flat-flat. Actually, I prefer a rolling contact on the last step. What was the population they were looking at (men or women, elite vs sub-elite, etc.)?
I've found that the flat footed penultimate permits greater lowering of the COM in front of the foot. I would think that there is a slight tradeoff in velocity because of it (due to the deviation in normal mechanics) but it seems to me that it's one worth taking.
It was men & women sub-elite but they said they had the mens 2000 or 2001 indoor LJ world champ (don't have the article in front of me). But the majority of the women were 5.x to low 6m jumpers and the men were mostly 6.5-7.5m jumeprs from what I recall. They note that one of their jumpers PB'd from 7.5 to 7.8x during the study.