I am getting 800cc round silicone implants. Can they be placed under the muscle? 5'4", 125lbs

Yes,  assuming your anatomic dimensions allow, the 800 cc breast implants can be kept "under the muscle" during the revisionary breast surgery. Generally speaking, the best online advice I can give to ladies who are considering revisionary breast augmentation surgery is:
1. Concentrate on choosing your plastic surgeon carefully. Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. Ask to see lots of examples of his/her work.
2. Have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining breast implant size/type/profile will most likely help achieve your goals. 
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as “natural” or "up a cup size” or "D or DD cup" etc means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup size may also be inaccurate. 
3. Once you feel you have communicated your goals clearly, allow your chosen plastic surgeon to work with you in determining the best plan to achieve your goals.
I hope this helps. Best wishes.

Silicone implants generally look and feel more natural than saline, and being sub-muscular (dual plane) is a better option than being over the muscle.  As other surgeons have suggested, you should probably think very carefully before increasing to 800cc implants.  Those implants will be very heavy and will place a significant load on the soft tissues, which will then stretch out over time.  You have a layer of scar tissue that has formed around your current implants that provides some extra strength.  I would recommend that you stay with an implant size that will fit easily within your pre-existing pockets to reduce your risk of complications.  Good luck!

Larger implants have a much higher revision rate. 800 cc implants are basically considered temporary. It is very difficult for the breast tissue and skin to support the weight. Such implants are generally removed in a short period of time, followed by downsizing and mastopexy (breast lift). I would recommend textured silicone subpectoral implants in general if large implants are used. Over the muscle placement will not give enough support of big implants long term. Good inframamary fold support is also essential. Other maneuvers, like raising the breast pockets internally (with a capsulorraphy) to get better upper pole fullness from a smaller implant -- to create the appearance of a larger implant -- may be a better long term solution.  Do your homework. Make an informed decision. Risks like a high revision rate or subsequent mastopexy may still be appropriate for your overall goals. Get second opinions if needed. Good luck.