A law firm could easily pay a number of app makers to try this out in exchange for the data, and then make a huge class action out of this.
I think the big challenge is that there are a number of confounding factors, like Apple's ranking algorithms.
In your test just cutting the 30% „Apple tax“, the developer would still profit from these discounted gift cards. But it’s not the developer giving the discount from his cut, but Apple from the 30%. Moreover, the 30% covers not only handling and processing App Store Infrastructure and sales, but as well the editorial sections of the App Store promoting news apps daily. There already exist reports on how this editorial content boosts app sales.
So, your test would still take all this for granted and therefore would be highly skewed. The data would not be „statistically significant“ at all.
You may buy iTunes cards. I've never bought them, nor even looked at them. I don't know which of us is more representative, but I don't think that invalidates the broad strokes of a test like I describe.
I'm not sure you really want law firms doing this. The recent practice of people investing into litigation has not exactly done wonders for anyone.
Instead, they have to make the indirect argument you do, and it's unclear what the harm is. They are not paying 30% more, the consumer is. There's no data to suggest they are selling less. Who is really injured?
The same issue arose in ticketmaster - outside of consumers, everyone else has a really hard time showing injury.
Meanwhile, the consumers, who directly pay the monopoly fee, are directly injured.