I doubt they'll allow us to customize companions unless we play as them. However, we'll be able to hire custom mercenaries, so that will probably be the way for min-maxers. For now, you can try the multiplayer trick to get a party of 4 Tavs, or just use mods to modify stats.
Incidentally, every companion except Shadowheart is using the recommended ability scores for their class, so I doubt their statblocks were made to reflect their characters.
