Will I see a big difference between 375cc and 415cc Ideal Implants?

Thanks for your question.  The difference in the two sizes you are contemplating is roughly 10%, which is not a major difference.  However, your surgeon will be best able to tell you which volume is most suitable for your body type.

Thank you for the question. Depending on the patient's body type and goals, a 40 cc difference in breast implant volume may or may not create a "clear difference".  For example,  for a more petite patient, even small increments in volume may make a significant difference in final outcome;  for "larger" patients, the same difference in volume may not be clinically noticeable.
Ultimately, careful selection of plastic surgeon and 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.
***Given that your surgery is coming up soon, I would suggest that you spend additional time communicating your goals/concerns directly with your plastic surgeon, 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.
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 "C or D cup” or "increase 2 cup sizes" 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 (and the attached link, dedicated to breast augmentation surgery concerns) helps. Best wishes for an outcome that you will be very pleased with.