It's pretty easy to see that "all DPS" is faster just by the limit break bar. Every time I've done "single group in" I've been able to LB2 2 of the stomach walls on the 2nd stomach phase (Black Mage). In "all DPS" method we usually barely have over 1 bar during the 2nd stomach phase. This also gives the off-tanks plenty of time to build aggro on the adds, so there's no danger of pulling hate when AOEing them down.