Honestly, not just with party size, but with other things I would love to see this, give players two alternatives to represent the two paths the mechanic could go and just let us mess around with both, and see what our opinions are with them.
A/B testing is certainly an effective tool, but for BG3 I think it is probably best employed as a survey/questionnaire; preferably offered from withn the EA game to obtain the widest set of opinions.
If, as you seem to suggest, they actually have to implement multiple options for multiple mechanics ( and possibly "balance" the game encounters for every combination of options ), that may not be an effective use of development time.
