Is 400cc smooth silicone implants above the muscle a good idea for my fit body type? (photos)

You small preop breast size puts you at risk for rippling after augmentation if you choose the subglandublar pocket placement.  I'd suggest a submuscular pocket to get more coverage over the implant.  

On a petite frame, implant visibility increases significantly.  When an implant >350cc is placed above the muscle, it is almost always visible, and you will notice rippling.  Highly cohesive silicone textured (gummy bear) implants would likely have fewer complications, if you want the implants placed above the muscle.