Implant width and impact on cleavage.
An increase in implant width of ~0.6cm is a minor increase. The extra width typically projects out laterally. That is because the skin over the sternum is quite fixed and unless surgically altered, the implant alone will not move into this area. However, the sides of the breasts will accomodate wider implants quite easily. Higher profile implants will likely give you much more cleavage than your current implants, but the distance between the breasts should not change.
Wider implants can help
diminish the distance between the mounds, as long as the capsule isn't released laterally. Some doctors are very reluctant to exceed the natural base dimensions of your breast so find someone that will do this for you. Your surgeon should be able to explain the anticipated results from going with a wider implant.
Widening the breast
Getting a wider diameter implant will make the breasts wider. You can't have it both ways it will be water on the side and water toward the cleavage. You must make a decision of what is most important to you, bigger breasts or wider breasts.
You seem to be on the right track to increase the diameter of the implant by 1 cm. The implant will most likely not go any further medially and a release of the capsule may need to be done laterally.
Will bigger implants make my breast look wider?
You are only going slightly wider with the larger implants that you mentioned. Chances are the differential in volume from 380cc to 505cc will add a lot more projection to the breasts which will make them stand off your chest wall more.
Will wider implants make my breasts closer together?
Because you already have implants, your pocket (capsule) is well-developed. When surgeons put new implants in, they usually cut the capsule towards the middle of the chest to move the implants closer together. This is because the implants always have a tendency to settle to the sides.
If your surgeon makes room for the bigger implant by cutting toward the outside of the capsule (toward your arms), then your implants shouldn't look much closer than they are now, especially because your new implant width is only slightly wider than your current one. An in person exam might help confirm this, but even then, surgical results are never 100% predictable.
Will change in diameter of implant to go bigger make my breast look even wider?
The width difference is 0.6cm so I guess there would be minimal change. But you need to discuss this with your chosen surgeon in person...
Changing volume of the breast without changing the profile the prosthesis will alter the diameter.
For a given profile increase in volume will increase the diameter of the prosthesis. The application of this to your particular issue is something you should discuss with your plastic surgeon.