I'm going to chime in here and say it's not that hard to balance a party of five versus a party of four. It may seem difficult, but There are exp tables and CR to determine the difficulty of encounters. And it's usually represented in a ration of per party member. Quite literally
It's not that hard to account for an additional party member when designing encounters in D&D 5e. The game is designed with the idea that DMs might have to put the encounter together at the last second. Some DMs even forego using the CR and tables, if they know the party's stats and abilities they can balance the encounter themselves.
The way Baldur's Gate 3 is balanced, adding one more party member would change very little. Adding two, sure maybe add in a few more goblins, duergar, etc.
TL:DR it shouldn't take significant playtesting since D&D 5e is already well play-tested.