I’ve had mine done (June 2010), and mine were placed under the muscle. At first, when the pectoral muscles contract over them, it feels weird. It will always feel differently than it did previously to the procedure, but you get used to it over time.
As far as ruining the investment, I think if you take your time and get back into the activities you once did prior slowly and carefully, you’ll probably be ok. Aerial fitness uses so much upper body strength, which will be impacted now that you have your musculature affected by your surgery. Implants themselves, depending on the type you have and where they were placed, are pretty tough and it would take quite the force to actually damage them. Also, whenever a foreign object is placed in the body, a scar capsule forms around it, adding another layer of support to the implant. All women with BA have some kind of scar formation around their implants. In fact, if the body gets too ambitious and overdoes the healing, you can wind up with advanced capsular contracture. I got back in to working out (heavy lifting, cardio, the whole bit) about six to eight weeks after my procedure. I’ve done crossfit type workouts, pole, plyometrics, running, and nothing yet has done my chest in so far.
Hope that helps!