I've been replaying SoA with a neutral-evil party primarily because of the Viconia romance and because of Edwin - if I just have six customized characters, no doubt they'll be stronger (and so blast through SoA and ToB), but you lose so much of what makes the story fun?

Don't really play BG1, but the same applies, you would think.

For sure in BGII / ToB. My ToB party has five pairs of boots of speed (entirely legally, that is); the sixth is my PC, a monk, who doesn't need it. Incidentally, monks seem to be the most overpowered of them all. My monk has a magic resistance of 93 as well as decent AC, excellent THAC0 and various other advantages like poison immunity, natural haste, hide in shadows/move silently (no backstab, but she really doesn't need it) and various special abilities that give tons of extra hits; between her and Sarevok I sometimes don't even bother to involve the rest of my party in fights. Turns out the 3 magic resistance per monk level caps out only after 25-ish levels; with a magic resistance amulet and the machine in Watcher's Keep on top of that, the result is near-magic immunity (plus excellent saving throws for the spells that aren't resisted). With enough healing potions to make up for the relatively low HP, she's close to unstoppable.

Should be doable in ToB, at least? Korgan and Sarevok for the muscle, Viconia for the healing, Edwin for the spells, plus PC. Okay, that leaves one slot for a neutral, but you can take one that isn't too goody good (personally, I keep Imoen in every party pretty much no matter what, but I guess she's not even neutral, so perhaps Valygar or so is better).

And more than that, with Greater Restoration. Ridiculous spell, honestly.