I honestly don't know if better arguments are needed. I think most people simply have no idea that this research is happening right now, and if they did the vast majority would be against it. Feels fundamentally different from stem cell or IVF.
We know that human brain cells in sufficiently large and complex assemblages are sentient (I don't know if the word "inherently" has any purpose here). We know this research aims at making the assemblages larger and more complex. It's a fundamentally different kind of argument IMO compared to those against e.g. stem cells or IVF.
(Slight aside but did you know that they're also doing tests on whole human brains taken from corpses, and they give them tranquilizers "just to be sure"? https://news.ycombinator.com/item?id=48212992)
> The same ideas were used to argue against stem cell research, IVF, and many other treatments and ideas that have saved and helped lives.
You write as if objections to (certain kinds of) stem cell research, IVF, etc. have been roundly defeated in some way simply because they've "saved and helped lives". This is naked utilitarianism and I hope I don't have to rehearse the reasons why utilitarianism fails utterly as moral theory.