Seriously. How can you make the argument a small set of people can give comments faster than the entire community? Its mathematically incorrect.
Firstly, the Demigods are not going to be balanced entirely on the comments of a handful of players - they'll post their observations, absolutely - but it's more about internal numbers; behind the scenes data mining. The less players they have to watch, the faster this collection can be completed. Why?
For the purposes of balance you need to see the new Demigods is all conditions otherwise you create situational Demigods - Demigods that are only 'balanced' when certain criteria are met, such as the presence of another Demigod, and this results in them being either over-powered or under-powered outside of their 'situation'. If the entire community has access to the Demigods, you'd see the usual rotational change that takes place when new content is added - namely, everyone would use them for a while, then go back to the Demigods they prefer if they don't prefer the new Demigods. This would result in the first week, possibly two weeks, being unable to be used for balancing because they'd mostly be playing in Mirror matches or games where everyone is using the new Demigods. This is fine for balancing in that situation, but you don't need two weeks worth of data for that - what you need to see is what happens when they play against and with
every Demigod, and multiple times. The more people who have access to the new Demigods, the longer it takes for the Demigod rotation to return to normal, where we'll see a healthy mix of all the Demigods so that data can be collected for, say, the Demon Assassin VS Oak, or Oculus VS QoT. Then they'll have enough information to begin implementing changes to balance them out.
If only a small number of players have access to the new Demigods, then the chances of them all playing in the same game decrease, and thus the likely hood of a game with a good spread of Demigods increases. As the chance for a spread of Demigods increases, the time required for data mining decreases as the odds of seeing identical matches also decreases. Thus, the less players have access, the less time is required for balancing. It's basic systemic data mining, it's not rocket science.
The Demigods are done. They do not need balance testing. The ONLY Demigod balancing that has EVER happened up until now is Erebus. That is ONE balance change. They are already a joke in this community for taking absurd amounts of time.
This is because the Demigods were balanced through the Beta and Internal QA quite extensively. This isn't because the guys and gals at GPG are magically able to balance everything on a piece of paper before implementation, it's because they take their time and do what is necessary to get it right. This point works for my argument more than your own, so thanks!