Hello OP... I'm the poor schlub who started that discussion you've been reading
I think the split difference is primarily noticeable when there's a $5 card that dominates for the player who gets to play it first (you mentioned Witch and Mountebank, Cultist is another). There are some strong 5/2-only combos too, like Mint/FG. But unless those cards are out there, 5/2 isn't as huge as you think it is. Put it this way... if your opponent's opening buys are Hunting Party and nothing, and yours are two Silvers, are you so far behind that you should resign right there? And even if his opening buy is Mountebank, if you open Silver/Silver there's a pretty decent chance you get yours on turn 3 or 4. After that, you just need your opponent's Mountebank(s) to miss one shuffle and you have a chance to even up the total Mountebank plays.
So if I'm saying all that, why did I suggest the feature in the first place??? Several reasons:
1) I also thought the opening split was a bigger deal than it probably is. The ensuing discussion, as heated as it got, taught me quite a bit about the game.
2) Isotropic had this option, other players had mentioned it, and I figured as an
option, it's cool for players who agree that they want as little variance as possible, say if they're in a tournament or other highly competitive context. I do like giving people choices. But it was
not something I thought should be default.
3) I thought my method of doing it with Salvager, instead of getting MF/Goko to provide the option, was a little bit clever and I wanted to get other programmers' opinions about it
Since you play primarily with a friend, you're obviously free to have any agreement you want about openings, but especially after the discussion in the other thread, I'd encourage you to play out those 5/2 openings for at least a few shuffles... sometimes they're as great as advertised, sometimes not. If it becomes clear that one player is dominating because they got to open Witch or whatever, you can always resign and start a new game. Even use the same kingdom if you're playing Casual or Unrated. No shame in resigning if you honestly think your chance to win is that low, but you shouldn't jump to that conclusion prematurely.