The people on the Source aren't more powerful than the people from shards for the same reason that 75% of the Ancient population is now in the aetherial sea to be reborn with their full, complete 14/14 souls and it will make little to no difference. I believe YoshiP described it as "maybe they'll be a little cooler, have a bigger mana pool." Their species has been eradicated. Their big thick souls will be stuffed into catboy and bunnygirl bodies, so it won't matter.