What should I expect with 275cc round Mentor implant BA? (Photos)

After I measure and discuss goals with my patients, we then look at pictures of women with similar measurements with different size and style breast implants. My patients tell me that this process is very helpful in determining what size breast implant is right for them. 
Dr. Ted Eisenberg, Board Certified Plastic Surgeon and Author                     
RealSelf Distinguished Hall of Fame Inductee

None of the modalities to approximate final results from breast
implants are 100% accurate; they merely facilitate the patient and the
board certified PS concurring on the size and type of breast implant
which will come the closest to reaching the patient's goal. You have done what my patients
prefer and that is to to try on sizers under garments they typically wear to determine
if they fit as expected and help allay some of their trepidation
associated with this decision. They usually bring a tailored blouse, tight sweater, swimsuit, workout clothes, tank tops,
and other garments they typically wear so that they have the best idea of
whether the garments fit as desired with the implant selected.  I would
recommend that you go with the size that boosts your self-esteem, and
fits your anatomy.
Choosing an implant based on an aesthetic goal that exceeds your
anatomic capacity may lead to complications and possibly revision. 
 Remember that there is no standardization of cup sizes between sizes
bra manufacturers.

Just based on your photo, I think you will have a great result that will look very natural. Please discuss any concerns you have with your surgeon.

Thank you for the question and pictures.  Based on your photographs, I think that you are starting at a good place and should have a very nice outcome with breast augmentation surgery. 
My best advice: more communication.
It is definitely worth spending more time (prior to the date of surgery) with your plastic surgeon to discuss further.  As you know, it will be important for many reasons, that you feel comfortable that you and your plastic surgeon are on the "same page" prior to proceeding.
Ultimately, careful communication of your goals (in my practice I prefer the use of goal pictures, direct examination/communication in front of a full-length mirror, in bra sizers, and computer imaging) as well as careful measurements (dimensional planning) will be critical.
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.
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. For example, I have found that the use of words such as “natural” or "full B, small C 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, the use of computer imaging has been very helpful during the communication process, in our practice.
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 intraoperative sizers.
I hope this helps. Best wishes for an outcome that you will be very pleased with.