I currently have Mentor 375cc smooth round high profile cohesive I implants- and would like larger! (Photo)

without a formal exam giving you a larger size would be irresponsible. It really boils down to your breast/chest dimensions, the type of "look" you desire and of course the amount of additional volume you are wanting to achieve. You will need to increase atleast 75-100cc for you to notice a significant change. 

Thank you for your question and photos. If you’re interested in going larger, I recommend you meet with a board-certified plastic surgeon to discuss your goals for what size breasts and look you are going for. Showing your plastic surgeon pictures of examples of what you would like your breasts to look like with help as well. Generally speaking, an increase in 100cc per implant will result in a noticeable increase in breast size. You can do a virtual consultation if you are from out of town. The plastic surgeon should show you before/after pictures to give you a better idea of what type of results you can expect to get. Good luck!

Thank you for the question and pictures.  Your concerns are not at all unusual;  many patients present  with similar concerns/goals. It should be possible to increase your breast size further with larger breast implants. Sometimes adjustments of the surrounding breast implant capsules are also necessary to achieve the desired outcome.
You will find that online consultants will not be able to provide you with specific advice in regards to selection of specific breast implant size/profile. Ultimately, careful communication of your goals will be critical.
Generally speaking, the best online advice I can give to ladies who are considering revisionary (upsizing) 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. I have found that the use of words such as  “natural” or "C or D cup” or "little fuller" etc 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.   
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).  Viewing the patient's chest wall in the upright and supine positions, with temporary sizes in place, help select  the best breast implant size/profile for the specific patient.
I hope this helps. Best wishes.