Thank you for including photos, and the comment that you "don't want upper pole fullness." I'm not sure what "prep" is, but you should take the suggestion that others have given and wear 800cc implants in your bra for a few hours and see what that looks and feels like. How did you and your PS arrive at 800cc?
800cc is over 3 cup sizes larger than what you have now, and even though you have some breast droop, your nipples are above your inframammary crease, and augmentation alone is all that is needed to give you a good result. However, adding over 3 cup sizes into your skin brassiere will undoubtedly give you a high, tight, rounded look that certainly adds upper pole fullness--that volume has to go somewhere. It may also give you an inframammary double bubble deformity as your implant causes a "new" lower inframammary crease below your natural one.
Furthermore, you should be considering silicone for the softest, most natural feel and to avoid the possibility for requiring re-operation if your saline implant(s) leak and deflate, which cannot happen with the latest generation of cohesive silicone gel implants. Below the muscle will help to keep the upper pole fullness down compared to above the muscle, and below the muscle also has the lowest likelihood of bacterial contamination (from intraductal bacteria) that can lead to capsular contracture (which will give you HARD, high, tight upper pole fullness).
Truly, I am usually the plastic surgeon on this site who champions patient choice--in coordination with the surgeon's assessment of your anatomy--regarding implant size, and I do use 800cc implants on occasion. I just really question your surgeon's judgement and experience if he/she condones, agrees, or promotes this choice for your anatomy. Have you shown your surgeon photos of patients or nude models with the breast size and appearance you desire?
Reschedule surgery and get another or two consultations and see if 800cc is offered by experienced, ABPS-certified plastic surgeons with lots of breast experience. If you can get one or more PS to agree with this plan, then you can always go ahead. But if not, you should realize there IS a reason that virtually all of your on-line experts are in agreement that this will be a huge mistake, and one that perhaps cannot be undone by simply "exchanging for smaller implants." You can always go bigger, but going smaller is much more problematic.
Good luck and best wishes!