As stated before, the area between the belly of the limb and the string is the stored energy. A longer draw achieves a greater amount of stored energy which can (all other things being equal) result in more (dare I say the word) speed and (here is a better word) momentum.
It is almost impossible to have the referenced 2 identical bows, one XX@28" and the other also XX@30" because something is different in the limbs to achieve the same weight at a different drawn length. But it is fun to hypothesize...
What is really impressive is some of the short bows I shoot. The I have a 56" bow that is nearly as fast as a 68" bow, both drawn 32"+. So, keep in mind, there are other factors influencing results...(design)
For those of you who are drawing "short", don't think for a minute what a long draw can achieve is so perfectly wonderful. In a long draw, all imperfections are magnified....