EPTFrom a Terran or Protoss point of view, I feel like a more expensive and more "able" overseer is somehow less of a threat than a dirt cheap and less "able" overseer, because it is primarily a mobile detector, secondarily a scout (changeling is cheap, overseer also faster than overlord) and thirdly a harassment unit (harassment because contaminate does not act as a battle support ability like fungal / transfuse).
Essentially, by buffing its tertiary role: the ability to harass, you're forced to weaken its primary and secondary role, which are much more important. Also if you consider the fact that Zerg players are getting pretty good at harassing, denying expanding and keeping enemy army size down, buffing their ability to hamper enemy production slightly at the cost of safety at home could actually have a negative impact on the use of overseers in general because their viability suffers.
