Is there a way to predict end results before moving forward with surgery?

If you were happy with the look of the 275cc implant you should be fine with the 300cc as this is less than two tablespoons of additional volume. In my practice I use sizers during surgery to help get the implant that fits the space best to get my patient the desired outcome.

The short answer is...no. Predicting the results with absolute certainty is impossible.
The choice of implant size/volume is a decision which is best made in concert with your board certified plastic surgeons. There are a number of factors which play into the decision including patient desire and chest measurements. Ultimately, your surgeon, based upon his/her exam will be able to guide a prospective patient toward reasonable options.
There are a number of tools available to help a patient make decisions with range offered by their surgeon including sizers, goal photos and 3D imaging. However, it is important to realize that these are "guides" and not "guarantees." They are meant to offer a patient a reasonable estimate on which to make decisions.
3D imaging has become an effective tool for discussing options for patients and narrowing down choices. However, it is important to remember that the imaging is only a simulation. There is no substitute for the experience of a board certified plastic surgeon.
It is vitally important to spend time discussing the results of your imaging consultation with your surgeon. As this will help clarify your goals/expectations and guide your surgeon in adjusting those expectations as needed.
With regards to your specific question, I would not struggle over small variations in volume. 25 ml is equivalent to 5 teaspoons and is about half a shot glass. This is unlikely to make or break a result. If your were happy at 275 cc, 300 cc will essentially look the same.
As always, discuss your concerns with a board certified plastic surgeon

I recommend an in person consultation with a plastic surgeon to discuss your goals and be measured. After I measure and discuss goals with my patients, we then look at pictures of women (my patients) with similar measurements with different size and style breast implants. They also get to see what they would look like when the breasts have fully dropped and fluffed and in clothes. You get to see actual results and the surgeon's experience and not a virtual idea of what can be achieved. Women tell me that this process is very helpful in determining what size and style breast implant is right for them. Dr. Ted Eisenberg, Board Certified Plastic Surgeon and Author. RealSelf Distinguished Hall of Fame Inductee. Philadelphia, Pa., USA