That's enough to say "let's not do this until we have better data".
If it turns out the second step is unknowable, that's enough to say "let's never do this".
These are subjective decisions, and some may suggest that there are gains that outweigh the potential suffering. I am currently unconvinced.
That is a Kantian, deontological approach, an ethical framework not shared by all. The utilitarian would ask how much suffering would be caused as compared to how much good it would produce.
Utilitarianism also comes with its own issues, of course. Exploitation of minorities and the disempowered, for example (including manufactured brains-in-vats, it seems).
The ethical questions raised by anomalous situations like this indicate that our historical decisions may need revising.
The unique data-point here is that we are creating these "mini brains". I'm not invoking a knee-jerk "sCiEnTiSTs pLaY g0d OMG!" reaction, but I will invoke Goldblum's Objection: "your scientists were so preoccupied with whether or not they could that they didn't stop to think if they should".
Like several others in this thread, I'm bemused that this passed bio-ethics. Drawing the line isn't entirely arbitrary, ethics panels do it every day. We already have the tools, but it seems that in this case we don't know whether to apply them, or which to apply.