Is it safe to 350cc of fat to each breast if I already have implants?

The amount of fat that can be transferred to your breasts depends upon how much donor fat you have and how much actual breast tissue is present. Implants do thin your breast tissue which may not hold as much fat as prior to implants.

The amount of fat that can or should be grafted, depends on how much tissue there is to graft into.
In my opinion, I generally try not to increase the volume by more than 100%.
In other words if somebody has 300 mL of breast tissue that I typically graft 300 mL of fat.
this is some degree subjective.
There are no yes or no absolutely answers to your question.
The results can be subtle, and there is a significant impact based on who you choose to do the procedure.
If the tissue is very thin between your skin and the implant, then only small amounts of fat can be successfully graft.
Best,
Mats Hagstrom MD