I currently have 730cc saline implants overfilled that are 11 years old. Getting them replaced Sept 4 with a full TT!

Thank you for the question. Unfortunately, online consultants will not be able to provide you with specific enough advice to be truly helpful.    As you can imagine, every woman has a different amount of starting native breast tissue, chest wall shape, body type... 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 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 "C or D cup" etc means different things to different people and therefore prove unhelpful.
 Also, as you know, cup size varies depending on him 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 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 (after the use of temporary sizers assessed with you in the upright position).
I hope this, and the attached link (dedicated to revisionary breast surgery concerns), helps.

I'm not sure if you misunderstood your surgeon, but the statement about silicone being narrower is simply not true.  Looking at the base diameters and volumes of different round silicone implants made by Allergan, for example, there are 800 cc silicone implants with the following diameters:style 10:  17.2 cmstyle 15:  16.2 cm (752cc is largest in this style)style 20:  15.3 cmstyle 45:   14.2 cmAs you can see, there is a wide range of base diameters, so if you really wanted silicone there would likely be a match for your breast dimensions.  You should revisit this discussion with your Plastic Surgeon to clarify things -- you certainly want to understand all of your options before surgery.

Implants shapes from saline to silicone are pretty similar. He may have recommended saline because he can overfill them.