275cc Vs 350cc Breast Implants for a Definite C Cup?

It is very important to communicate your size goals with your surgeon.  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 cup” or "fake looking" 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.
I use  intraoperative sizers and place the patient in the upright position to evaluate breast size. Use of these sizers also allow me to select the press implant profile (low, moderate, moderate plus, high-profile) that would most likely achieve the patient's goals. The patient's goal pictures are hanging on the wall, and allow for direct comparison.
I have found that this system is very helpful in improving the chances of achieving the patient's goals as consistently as possible.
By the way, the most common regret after this operation, is “I wish I was bigger”.
I hope this helps.

275cc Vs 350cc Breast Implants for a Definite C Cup?
. I would advise proceeeding with  a 350 in order to ensure a C cup but this is not a warrantly of results,

You asked a great question that a lot of breast implant patients are asking. First, the C cup needs to be better defined. The cup volume of a 32-C vs. 38-C are quite different. In fact, it's not always easy to find 32-C or 32-D cup bra. Based on your brief physical description, it appears that you want more volume and projection by considering 350cc high profile implant. With this choice, you are more likely to achieve a "augmented" look compared with 275cc moderate plus implant.
If this is what you want, then it's fine. If you wanted to have a more "natural" look, then consider the 275cc moderate plus. You did not mention whether the implant is saline or silicone but you did mention that implant will be placed above the muscle.
For saline implant above the muscle, you have to consider the possibility of rippling when you choose these implants for your body size. Another consideration is skin tone. If your breast skin is very tight then the larger implant will stretch the skin that much more.
There are a lot of issues when it comes to choosing implant size and shape. Make sure that you are completely comfortable with YOUR treatment plan between you and your surgeon.