Tbh, I like the explanation that they genetically engineered it into that 'design' of Vorta, but not others.
We know the Founders are seen as gods to the Jem Hadar (not just to Vorta), and yet the Founders still chose to make them reliant on ketracel white, just in case they overcame that 'programming' and rebelled.
Seems completely logical that they'd also limit the Vorta's abilities in order to more easily curtail any future uprising too. Giving them all the ability to fire energy blasts could be something the Founders regretted later on.
It's also true that the Federation's first meeting with the Vorta was all orchestrated. It's possible the energy blast is something they purposely show off during first contact to other powers, to act as a show of force to them.
As for plot lines that went nowhere in DS9, does anybody remember that there was initially a rift between Jem Hadar created in the Gamma Quadrant and those created in the Alpha Quadrant? It was shown in that episode where the defiant shrunk, but then was never mentioned again. They were clearly gearing up for a civil war plotline, but it never came to be.