What is Really the Difference Between a 450cc and 500cc?

Unfortunately, you have not provided any photographs or told us your bra size.  A 450 or 500 cc implant is quite large.  This may not even fit under your muscle.  Implants this large often dislocate inferiorly and laterally.  I recommend patients undergo the smallest implant possible to give the appearance they want.  There is a new technique called Breast Augmentation with Mini Ultimate Breast Lift, which reshapes the breast tissue, elevates it higher on the chest wall and increases cleavage.  Aligning the areola, breast tissue and implant higher on the chest wall produces more projection than traditional techniques.  This allows the patient to use a smaller implant.  Smaller implants will not descend as soon or as far as a large implant such as 450 or 500 cc.  Also, Breast Augmentation with Mini Ultimate Breast Lift technique uses only a circumareola incision, avoiding the ugly vertical scars that can not be hidden.

Gary Horndeski, M.D.

There are several ways to choose breast implants. One tried and true is "volumetric sizing" or trying implants on in a bra. When the implant is placed in the breast, under the muscle, you will loose some of the visual effect compared to the sizer in the bra. To compensate for this, we recommend using an implant that is approximately 10% larger than the sizer.

Thank you for the question. 
 
In my opinion, it is not possible to accurately determine how many ccs (if any) a patient would “lose” when undergoing sub muscular breast augmentation surgery. I suggest patients do not make decisions based on statement such as: “you will lose 25 or 50 cc of volume when breast implants are placed under the muscle”. These statements are simply not reliable enough to utilize clinically. 
***Given that your surgery is coming up soon (and your ongoing concerns), I would suggest that you schedule additional time to spend with your plastic surgeon communicating your goals, preferably prior to the date of surgery.
Generally speaking, the best online advice I can give to ladies who are considering 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. Sometimes, it is necessary to seek several consultations before you feel comfortable about your choice.
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. Again, best not to discuss your goals and/or judge the outcome of the procedure performed based on achieving a specific cup size. The use of computer imaging may be very helpful during the communication process. The use of in bra sizers may also be helpful. In other words, use as many “visual aids” as possible during the communication process. I encourage patients to meet with me as my times as necessary, to feel comfortable that we are both on the “same page”.
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. I generally select appropriate breast implant size/profile after the use of temporary intraoperative sizers and viewing the patient's chest in the upright and supine positions.
I hope this helps. Best wishes.

The size of the implants are based on a combination of a patient's goals
and objectives and her anatomy. In order to provide a natural looking
augmentation, the surgeon needs to evaluate many factors including soft
tissue coverage, skin laxity, chest wall width, breast imprint width,
the shape of the breasts and relative level of constriction to ensure
that the breast implants are appropriate for your body. Consult with 3
experienced and expert board certified plastic surgeons to understand
your objectives.

If you are going under the muscle and getting a lift, I would recommend at least 500cc to meet the look of the 450 cc sizer.   In order to make an accurate size recommendation, I would need to assess your chest wall and breast mound measurements and characteristics.  Unfortunately, there is not a general rule of thumb or objective criteria to implant selection.
Your plastic surgeon will perform several measurements of your chest wall and breast anatomy and determine a range of implants that both fit your chest wall and reach your desired goals.
The next step is to try on this range of implants in the office with your doctor.   The key to this success is showing your surgeon the body proportion you desire with a bra sizer and allowing your surgeon to guide you to the right implant.   It will be much easier to communicate in implant cc's than cup size when determining the appropriate implant for you.
I wish you a safe recovery and fantastic result.
Dr. Gill