After testing party of 6 with a few more encounters, the way I see it, there are two ways for this game to go:
1. Larian wants the majority of the fights to be tough, so they will tweak the encounters so that players will have to long rest more frequently, after almost every fight, in order to continue. In this scenario, a party of 6 will definitely lower the frequency of long resting. Fights will be less deadly and therefore the party will have to long rest less. They could essentially move through pretty much most of the surface of EA possibly without long resting once, depending on how good the player is. So, if this is Larian's intent, then yes. A party of 6 will certainly require not only using proper 5e stats but also require adding more enemies to various encounters in order to compensate for the 2 additional party members. Certain fights, however, like the wood woads, would only require 5e monster stats to be implemented. 2 wood woads alone could handle a party of 6 level 3-5 characters. Add mud mephits as well, and it'd be a tough fight. Currently, this fight is only doable with party of 4 because the monsters are severely homebrewed. Either way, if Larian is going to stick with fights that are deadly almost every fight, and they want players to long rest frequently, then a party of 6 will certainly require some additional enemies per encounter for most fights.
2. Larian wants the majority of fights to be moderate to easy with a few that are really tough. In this scenario, the idea is that the players will try to long rest as infrequently as possible. Therefore, they will fight their way through the surface of EA, plowing their way through enemy encounters that more slowly chip away at their resources as opposed to fights that leave them virtually unable to continue without a long rest. In this scenario, the player would be able to clear more of the map before needing to long rest, so it would fit a bit more with the game, actually. You have a tadpole in your head, so you want to sleep as little as possible. You have characters in your party urging you to move your butt to one destination or another. You have a ritual being completed that is supposed to boot everybody out permanently, and so forth. So, storywise, you are racing against a clock, even if mechanically you are not. So, being able to clear more of the map without long resting goes along more with the story. This said, fights may feel a bit too easy as you work your way through. The fights won't all be quite so "edge of your seat." On the other hand, you also would not have as many situations where you have to reload because you lost initiative and the enemies slaughtered your party before you even got a chance to act (I can't tell you how many fights where that's happened to me.)
So, yes, if Larian wants a constant "edge of your seat" experience from battle to battle as you slowly and painstakingly make your way through each encounter, then a party of 6 would require a good amount of reworking encounters to add more enemies, etc. If they want more of an "each encounter will wear you down until you need to long rest" experience, then a party of 6 plus proper 5e stats will do just fine.
All this is to say that three things would be required, in my opinion, to make us party of 6 players happy:
1. Give us the option to play with 4 custom characters in single player mode so we can have, at minimum, a party of 4. This would make the prologue more of a party of 5-6 experience once you add Lae'zel and/or Shadowheart. 2. Give us the option to play with a party of 6 max. 3. Give us the option to play using proper 5e stats and rules.
After testing party of 6, I think these three things would make a party of 6 experience fun, feel more like we're actually on an adventure and not such weakling idiots who can only travel for like 5 minutes before we need to sleep the rest of the day away, and we'd experience all the dialogues and party cohesion that a party of 6 provides. Besides this, if playing 4 player multiplayer, allowing us 2 additional origin characters means that we can still do all the side story quests and such that we would be locked out of with a party of 4 max.
Give us these three things as options, and I think we'd be all set. The "edge of your seat" per encounter crowd would be happy because they could still play party of 4 with current encounters as is, and the party of 6 crowd would be happy because they would have a less "edge of your seat" per encounter experience with party cohesion and a more genuine RPG adventure experience like you might find in a standard tabeltop session.
And don't get me wrong. You'd still have a few encounters that were edge of your seat with a party of 6 and the other 2 options I mentioned above. Again, 2 wood woads with proper stats plus a few mud mephits would be a hard encounter for a party of 6. Ragzlin with proper hobgoblin warlord stats plus his room full of cultists and such would be hard for a party of 6 level 4 characters. Minthara with proper AC would also be hard, along with her goblins and spiders and such. The gith patrol is still brutal, regardless of 4 or 6 party size.