Would 650cc overfilled to 700 (or 750) under muscle be enough? 225-250cc doesn't sound enough to make a visible difference.

Based on your pictures and desires I would suggest nothing bigger than a 650cc. I would also use siliocne gels and not saline. I would caution you against the use of saline when going this big. Saline implants are notorious for wrinkling or rippling. Your chances of having visble and palpable unsightly and undesirable ripples increase incrementally as you go up in the ccs. In addition, you are very slim and therefore do not have enough soft tissue coverage to be able to camouflage the rippling which is sure to happen. Good luck

Thank you for the question and pictures.   Despite good intentions, online consultants will not be able to provide you with  meaningful specific advice.  Ultimately, in person consultation with well experienced board-certified plastic surgeons will be necessary. Careful measurements, dimensional planning, and careful communication will be key.
Generally speaking, the best online advice I can give to ladies who are considering  revisionary breast augmentation surgery ( regarding breast implant size/profile selection) 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're 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 which operation and/or 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 "C or D 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.  The use of computer imaging may be very helpful during this communication phase.
3.  Once you feel you have communicated your goals clearly,  allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals.  Again, in my practice, this decision is usually made during surgery. 
Remember that both you and your plastic surgeon should emphasize safety and avoidance of complications above all else. I hope this, and the attached link (devoted to “extra large” breast augmentation), helps.