I've played with a six member party with a mod. It's not cumbersome at all. It does obviously trivialize the current encounters even more though. That means, Larian would have to rebalance everything again.
It won't happen. The game's bones/core is essentially done at this point. Larian wouldn't market Early Access where core game features can be altered. They have built this game for a party of four and that's what we are getting.
OR, since this is Act 1 it can be allowed to be a bit easier and they only need to balance content beyond Act 1? And I think the point of this EA is so they can modify core features, they have been trying to fix the RNG system for example which is a core game feature.