Can a 800cc saline Implant Be Overfilled to 1200-1500cc to Have a Rounder Shape?

To answer this question:  Yes, you can fill an 800 cc implant to 1200 cc  to achieve a rounder shape but the more important answer question to ask is : Should you fill it to 1200 cc. The answer to that is NO! This will likely produce a very round implant with scalloping of the edge and at higher risk of mechanical failure.

First, I do not recommend 800 cc implants and to overfill them by that much may be too problematic.  I do not know of any surgeons that would do that.

There afre several ways to get the fake look-either with overfilled saline implants, custom XL silicone implants or XL tissue custom expanders.It depends really on what cc size to want to get.
You can think of a saline implant as a balloon. The manufacturer recommends an overfill of about 20cc but they have been shown to be able to be filled much more than that. They do start loosing their shape and can significantly distort the appearance of your breasts. 
 I do overfill espcially since the FDA has restricted saline and silicone implants to a maximum saline shell size of 800cc and maximum silicone size of 800cc. I do not know why that number was picked arbitrarily. 
So, I do do breast reconstructions greater than that with XL tissue expanders which do pride a greater diameter and projection and upper pole fullness and allow the size to be adjusted either up or down. Also, the shell of the saline implants are very thin and can dreflate more easily. You may want to visit my site below which explains everything, esp for XL sizes which is my specialty, either initially or revisional surgery.

This can be done but it is not recommended.  The moderate plus mentor implant can do up to around 1000ccs.