The two guys who were running hardest over the last half both used some of cooling strategy before the race. Meb and Deena both did the same thing before their Olympic medals. Staying cooler longer = faster. I would bet that running without a shirt would keep you cooler for longer than running with a shirt due to evaporative cooling, and a shirt with holes in it is closer to running without a shirt. Makes sense to me. Not sure how well those bibs would have fit on a half shirt. I will say that jog bras are cooler than singlets, although they kind of restrict the chest. Along those lines, Reggie Lewis Athletic Center does not allow shirtless running, but jog bras are allowed.