Are 600cc Breast Implants Too Large?

There is no way to quantify your risk. Certainly the risks of complications increase as the size of the implants increase. In general there is not a big difference between 420 and 600cc but you should at least try a sizer on.

The simple answer is yes. Being  5'11' and 140lbs you are a 19 BMI which means you are thin. You said you are high and round which means you may have some capsular contraction. This means that you have to get a complete circumferential capsulotomy to be able to place a 600 cc implants. You really should have a discussion with your surgeon who knows all the details. For example the quality of your soft tissues, your skin, your chest dimensions, etc.

Thanks for your question -
As the other surgeons have mentioned - it is difficult to answer that question.
Generally, the larger the implant the bigger the risk of needing a lift later, having back pain, etc. Also, generally the larger the implant the less natural the look.
You do have the benefit of already having 420 cc implants so you're already familiar with the general appearance of an augmented breast. The one thing I would caution is typically a high profile 600 cc implant will look substantially different (and likely less natural) from a lower profile implant of similar volume.
Spend a fair amount of time trying on sizers or rice and speak with several plastic surgeons.
In the end, it is always your decision, just make sure you have a surgeon that can explain the possible risks, benefits and alternatives. I hope this helps.