Going from an A cup to 400-450 cc

will show you what a 450 cc implant will look like in you... use a sizer around 400 cc's in an unlined bra and this is just about what you will be with the 450 cc implant in your body.  Is that too big?  Only you can decide and I only intervene when I believe patient's are starting to look foolish... I do have a reputation to preserve and simply putting in ginormuous implants does nothing for that.

At the end of the day 50cc's is nothing with regards to size. What you should be focused on is that you have chosen a surgeon that is going to safely put the correct implant that fits your body frame. There are pretty basic guidelines to choosing an implant for a patient. There are about 3 profiles you can choose from. The main idea is that your surgeon measures the width of your breast and chest wall and picks the implant that stays inside those parameters. If you choose an implant that is too wide that is when you get the so called "side breast". So first, know what the width of your breast is, then look at the different profiles within that width and choose the one that gives you the look that you need. Again, 50cc's either way is a drop in the bucket. Usually I don't see major changes until I get to the 100 or more cc increments.

450cc into an A cup will make a huge difference and for your given height and weight I can assure you that you will not look natural, whether you choose silicone or saline. If that is the specific look that you desire, that's absolutely fine. However, I would personally agree with the people who have told you that you will look weird and too big, and disagree with the doctor who said it won't be that much of a difference. 

Try to find a surgeon that can do 3D analysis with a VECTRA 3D camera.  The 3D image the camera produces can be analyzed with sophisticated software that can then simulate any size implant with a 3D image and how it will look on your frame.  Implants really need to be matched to the base diameter of your breast or else they can be too wide or narrow for your frame.  Also, implants that are too big might look great for the first few years, but over time can thin and stretch the tissues causing them to bottom out.  This can be a really tough problem to fix.

It's very hard to answer questions about specific implant sizes without an exam but at your height and weight the 450cc will probably get you somewhere between a full C and a full D.

It's almost impossible to go from an A cup to more than a small C in an hour or 2 hour surgery.  You only have a certain amount of skin around your chest to work with.  Women that get large with breast implants do that in stages.  Try on sizers in your surgeon's office prior to your surgery while discussing volume.  Best wishes!