I have 425cc and thinking about upsizing to 800cc

Without seeing your photographs, this is a difficult question because typically we place implants that are based on your anatomy and your specific proportions. It is absolutely possible to place an 800cc implant but I would never do this without some type of soft tissue support scaffolding such as Galaflex, for example. The Galaflex can be used either as a lower pole sling/internal bra supporting just the lower pole or it can be used as a complete off label wrap providing 100% coverage of the device with improved support, integration, and maximizing upper pole fullness. I have done this for many years and am able to get predictable outcomes without fold malposition even with such a large implant.

Dear Brave257807,
breast implant type and sizing selection is a complicated process and requires a detailed consultation that includes a discussion of your goals, a detailed examination, and a lengthy discussion. But the decision will always start and end with the patient. When it comes to sizing, we have all of our patients find 3 photos of their desired breast size. We ask for naked breast photos because clothing can obscure the true size (push up bras). Next, we consult with patients and go over the photos and take detailed breast measurements during a physical exam. Next, we using a sizing algorithm to determine a range of implants that will work based on the exam and the measurements.Lastly, during the operation, we use silicone sizers in the operating room to see how the size actually looks in the breast. We sit the patient up in the operating room, while asleep of course, and compare the sizer result with their desired look photos. If it is too small, we start the process over with a slightly larger implant. We don't stop until we get the perfect match.Of all of my breast augmentation patients, every one of them has been happy with their size due to this detailed and meticulous process. You don't want to invest time and money into a result that is either too big or too small.You should discuss your concerns with a board-certified plastic surgeon and make sure that your surgeon understands your goals.
Daniel Barrett, MDCertified, American Board of Plastic SurgeryMember, American Society of Plastic SurgeryMember, American Society of Aesthetic Plastic Surgery

Just have in mind these facts: every 150 is a cup size. If you are DD you are adding 3 cup sizes.
Also you are carrying now almost 2 lbs (800 gr) on your spine, after you willl carry 4 lbs. (1600 gr) and your breast tissue and skin will only became saggier because tissues are made of iron.
So all our decisions have consequences and we all have to live with them.
Dr. Cárdenas

It is difficult to predict bra size as there are many factors that determine bra sizing. Every company has their own sizing as you may size one way in a particular brand and then another size with another company.
In general most would agree that 180 cc or somewhere in that vicinity will affect the bra size by one size. Now that also depends on the patient, their body and so forth.
Hope that helps.

Why do you want this? Please do not go up to 800 cc's. Those implants are much too large and heavy for your neck and shoulders and will thin and stretch your tissue. Your breasts will sag from the heavy weight. consider something like 500-600 cc's. However, you will need to be measured to see if this is possible. Please do not ruin your breasts.