If they suddenly shift to a 6-person party - given that all encounters are presumably intended for 4-person parties - how can this not affect encounter design/difficulty? Surely encounters will be too easy so you'd need to rejig them?

I have no issue with a party of 6 just saying that I do think it requires additional work: I don't think that simply levelling up more slowly for a larger party would 'auto-balance' encounters. You can't know that without play testing, and the vast majority of testing has been with 4-person parties. The game is already quite doable with 4 or less PCs. 6 would make it a cake walk, all those extra turns would make a huge difference, even if the characters are lower level. And people would complain about how easy it was. I think they need to decide on what the canonical party size is and design around that. Maybe that should be 6, given the history of BG, but I don't have a strong opinion on this and it's not a deal breaker for me.