Is Strattice safe? Required? Symmastia repair & prevention. Can I go 650-700cc over the muscle?

It seems to me that you are describing quite a few issues that are often linked with implants that are too large for someone's body frame or skin envelope. While there certainly are surgeons that are more skilled than others, I would suggest that most implant issues have less to do with surgical technique and surgeon talent and more to do with poor surgical planning and an understanding of physics and the natural laws of gravity. While it is possible that your surgeon made your pockets too big, it is also possible that the pockets were made properly, but that your implants are just too big for your soft tissue and connective tissues. Its probably not what you want to hear, but I would suggest exchanging your implants for smaller ones and not larger. The pockets can be improved with capsulorrhaphy sutures to stabilize the position of the implants. Strattice and other support materials are safe to use, but probably are not needed if your implants are more proportionate to your body frame.

I am sorry to hear about your concerns after breast augmentation surgery.  
 Although it is possible to have breast implants replaced in the sub glandular position, I do not think that it  will be in your best interest to do so.  Generally, breast implants placement subglandular position can be problematic;  concerns such as rippling, palpability, encapsulation,  accelerated "sagging" of the breasts…may arise.
 Given your description and, based on your photographs, revisionary breast surgery involving correction of implant displacement (capsulorraphy),  judicious adjustment of the breast implant capsules (capsulorraphy), and possibly the use of acellular dermal matrix may be indicated.  I utilize Strattice frequently and have not had any problems with its use.In my practice, having used a variety of “techniques” for correction of symmastia or medial malposition, I find that the most reliable technique involves a 2 layer capsulorrhaphy (internal suture repair of the breast implant pockets along the cleavage area). Often, the use of acellular dermal matrix is helpful also. I have also been pleased with the (at least partial) correction of skin tenting that can be achieved with the use of the acellular dermal matrix.As you mentioned, careful selection of appropriate breast size/profile is also important.  I also find that careful attention to postoperative activity restrictions is key to success with this type of surgery and other types of implant malposition revisionary breast surgery.  Activity restrictions are individualized patient by patient, depending on exactly what was found and done during the procedure.  For example, for most of my symmastia patients, I ask that they avoid pec major related exercises (especially exercises that involve abduction of their arms) for at least three months after the procedure is performed.Nothing will replace careful in person evaluation of your concerns/goals when it comes to providing you with accurate diagnosis and treatment recommendations.
 Best wishes.