Getting 600cc HP gummy bear implant under muscle with a lift. Does moving the implants under the muscle help lift them higher?

Yes, generally speaking, breast implants placed in the sub muscular position tend to "sit" higher on the patient's chest wall.   Besides breast implant positioning, there are many variables that come into play when it comes to the final aesthetic outcome achieved with this type of revisionary breast surgery.***Given your concerns, it is definitely worth spending more time (preferably prior to the date of surgery) with your plastic surgeon to discuss further.  One question that might be helpful to you: will you be more disappointed if you are slightly too small or slightly too big?  It will be important for many reasons, that you feel comfortable that you and your plastic surgeon are on the "same page" prior to proceeding.
 Generally speaking, the best online advice I can give to ladies who are considering  revisionary 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. For example, I have found that the use of words such as “natural” or "C or D cup" or "much fuller breast" 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. Again, the use of computer imaging has been very helpful during the communication process, in our practice.
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.
I hope this helps. Best wishes for an outcome that you will be very pleased with. 

Thank you for the photos and question but an examination is really needed.  That said from the photos alone you do not appear to need a lift but going larger and submuscular should give more upper pole fullnessDr Corbin