Yup! Flats will be done much sooner. In part, that is why I've never cooked a flat.
A packer trim brisket, with a "nose", and with a nice fat cap is the way to go, IMHO. Cook that low and slow, with the fat cap "up" is the conventional wisdom. However, some of the brethren have been "speed cooking" briskets.
Smoking the brisket for a good 17-22 hours has given me great results, with the fat rendering off and basting the meat on the way down, turning it into a truly glorious tender state. And the nose? WOW! I find myself picking bark off that part of the brisket before anyone else gets to it, under the "cook's privilege" rule.
Also, flats tend to be a bit more dry, again, because there is not much by way of basting going on: Most flats have much of the fat cap trimmed off.
Brinkmann Cimarron (Binkie)
Weber Performer (Fudo) with Smokenator 1000
Outers electric cold smoker
Lump in the soles of my shoes
My avatar is M-57 the "Smoke Ring nebula", in memory of "e".