Basically, Steph with surgery has a different storyline to no surgery Steph. Similar to office slut Ellie vs non-slut Ellie, and whilst that might be true in the real world, there have been massive advances in technology regarding surgery in DLC's world.
So whilst there will be two sets of renders for Steph, they'll be completely different scenes. You won't be able to get Bimbo Looks Steph scenes without modifying her, and you won't be able to get non-Bimbo Steph scenes if you do. Yeah it is extra renders, but I like having choices actually change the game, same way you can have innocent Ellie or slutty Ellie in v.10.