Going from 750 Cc's in Tissue Expanders to 650 Cc's Silicone Implants?

That sounds very reasonable to me. Just keep in mind. The expanders will always be less projecting than those stiff expanders. Silicone will project less than it's saline counterpart as well. I hardly ever use saline in breast reconstruction. I only use it when pushed into it by a patient out of fear of silicone which is rare. I would not get caught up on cup size since bras vary so much. From a breast reconstruction surgeon, I try to make new breast mounds that will fit the body. I never go for a cup size. It is great that your radiated side expanded. I have had success with expanding some radiated breasts like in your case. So, I think that is a reasonable choice in size for your surgeon to be considering at this point. Things can change in the operating room however, and that is why I use sizers before I choose a final implant. Good luck! You are one step closer to ending this long journey.

I personally place larger silicone implant size compared to tissue expander size.  Are you happy with your current tissue expander size?  If you are happy with your current expander volume, I would go slightly larger than that.  Best wishes.

No, you will be 100 cc's less than your tissue expanders based upon your description. It may very well put you in a bra close to what you are wearing now.