Is there much size difference between 275cc moderate plus or 300cc high profile?

The styles are similar and the volume difference is small, less than 2 tablespoons. I recommend an in person consultation with a plastic surgeon to discuss your goals and be measured. After I do this with my patients, we then look at pictures of women with similar measurements with different size 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

Hello and thank you for your question. Honestly, a formal in-person evaluation would be needed to determine the best treatment. There are several reasons for this:
1) Bra sizes vary based on the clothing store (for example, Victoria's Secret tends to upsize everyone by a whole cup size)
2) Most women do not wear the proper exact fitting bra size for their bodies
3) The exact volume (in cc) needed to change cup size is different for different body types (e.g. 32A to 32D requires a different implant size than 38A to 38D)
4) Be careful NOT to fall into the trap most women do- they see a certain implant size (e.g. 285 cc) or perhaps their friend had a certain size, and they want or request this or another size based on that. Everyone's body is different- 285 cc implants on your friend might look VERY different than they would on you. Be very careful not to make this mistake, because the goal is to find the right size and shape for YOU- for your particular body and your breasts.

Because of those multiple factors, and because it is KEY to fully evaluate your breasts in person to make a full set of precise measurements to select the proper implant for you (incorporating factors like amount of breast tissue, implant type, base width, projection, etc), answering your question really would need a formal personalized consultation- to say anything before that would simply be guessing, which would be both unfair and unhelpful to you. Also, just FYI, each surgeon's measurements are slightly different, so numbers, experience, and overall aesthetic from one surgeon aren't necessarily the same as those from another surgeon!

Your breasts are such a KEY area of your body and your femininity- therefore, you should ONLY trust an experienced aesthetic plastic surgeon. As such an expert, I have had extensive training and experience with complex aesthetic surgeries under a multiple world-renowned experts in the field. I’d recommend for you to setup a formal consultation with me (or another plastic surgeon who is extremely comfortable with aesthetic plastic surgery procedures) for a formal consultation to thoroughly discuss your surgical goals, undergo a formal examination, evaluate all your options (surgical and nonsurgical), and decide on the best procedure(s) and type of anesthesia for your specific case. I hope this helps!

Dr. Donald Groves
Aesthetic Plastic Surgeon

Not much difference...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.
One question that might be helpful for you to consider: would you be more disappointed if you are slightly too big or slightly too small after the breast augmentation procedure?
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 fake look” 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, 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.