What cup size will I be if I started out as 34c with 600cc silicone high profile implants?

At size 34 C each 100 cc’s of implant
corresponds to 1 cup size change.600 cc
implants will take you up 6 cup sizes to a size I, are not stable, weigh over 1
lb each, will not fit retro-pectoral, will extrude inferiorly and laterally
requiring revision.I always recommend
small round textured silicone gel implants placed retro-pectoral since they
look and feel more natural, are more stable, less likely to ripple or have
complications needing revision.Implant
profile is irrelevant in the retro-pectoral position since the muscle compresses
it.You need to reconsider your implant
volume size.Best Wishes,Gary Horndeski, M.D.

There is no way to guarantee a particular bra size. Bra sizing varies greatly between bra manufacturers and a C in one bra will be a D in another. What matters more than the assigned bra size is the way the implant looks on you. The best option for your body and aesthetic goals can be determined in a thorough implant sizing session.Implant sizing depends on several factors. One of the most important factors is your breast width. Generally, your surgeon will measure your breast width, and then provide you with a range of implant sizes appropriate for your native breast size. There are more nuances to it than just what I've described, but this approach works for most women.I usually have my patients bring in a large bra and a tight t-shirt to do sizing. I'll then choose 3-4 implants that I feel are appropriate, and have my patients place them in the bra under the tight t-shirt. My patients can then look in the mirror and get a good sense of what they will look like with the provided implant sizes. My patients like this approach and get a great idea of how they will look.By using this technique, your surgeon can outline a range of appropriate implant sizes that will be aesthetically pleasing, and you make the final decision.I hope this helps. Good luck!

It is difficult to predict the final cup size after adding a volume because bras do not always match up to volumes. Roughly you are probably looking at an increase of 2 possibly 3 cup sizes.

Thank you for the question. There is no accurate correlation between size/profile of breast implant and cup size achieved. I ask patients not to base their communication of goals, or evaluate the outcome of breast augmentation surgery, based on achieving specific cup sizes.
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. I have found that the use of words such as  “natural” 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.
 The use of computer imaging technology may also be very helpful during this communication process.
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.
I hope this, and the attached link, helps.

Generally speaking an estimate is that 150-200 cc of breast implant will enlarge the breast by 1 cup size.Using the bra that you currently you that is a C cup then you should increase by at least 3 cup sizes after 600 cc breast implants.600 cc breast implants are huge in my opinion and rarely look normal and most often look fake and distended.  In addition your risk of complications such as capsular contraction and others increases with implants above the 450 cc range in my opinion.