Breast Augmentation with Prominent Sternum?

Yes, it can be difficult to create sufficient cleavage in patients with prominent sternum (pectus carinatum) following breast implant augmentation surgery. Furthermore over time the breast implants tend to slide outwards and down the chest incline. In these patients, I frequently advise them to wear nocturnal support garments to minimize this tendency when supine

It sounds as though your plastic surgeon has a good plan for you without actually meeting you in person. Breast implants are wonderful for improving cleavage while increasing the size of your breasts. There are many different ways of putting in the implants, so make sure you review this with your surgeon. Also, breast augmentation has very few bad potential complications, so you should be okay in the short and long term. Check out some before and after photos before you make your final decision. Good luck!

Your condition is called Pectus Carinatum ( latin for "bird like" chest")
A lot of good advice has been given on this forum, I would only add that the best option for you will be to use a High Profile implant.
The high profile implant projects more from the chest wall, counteracting the effect of the prominent sternum.
I hope the link I provide below will help you understand the whole issue of projection and chest types a little better. It goes into high placed implants versus medially placed implants, both of which are referred to as having "good cleavage"
You may not be able to get a super defined medial cleavage, but with a high profile implant and proper placement (higher?) you may get closer to what you want.

There's nothing that can be done about your prominent breast bone, at least nothing reasonable. You need to have a standard breast augmentation done and your breast bone will still be prominent. If your nipples point a little to the sides, they'll appear to point a little more towards the sides just as a natural effect of the surgery. Remember that the augmentation will enhance every aspect of your breasts (both good and not so good). Ultimately, you'll still be very excited about your result since the augmentation factor is no longer of concern.
You can't really place the implants any closer together if they're being placed under the muscle. You don't want him to split the muscle along the center just to satisfy you insisting on cleavage. I also don't think you should get a bigger implant to cause cleavage. Cleavage is usually something you see when someone has a bra on and it's pressing the breasts together. It's not really because of a certain style, position or size of an implant -- so you should reconsider changing your implant with that intention in mind.
You should revisit with your ABPS board certified plastic surgeon on these issues and clarify your expectations so everybody is thrilled after surgery.

If your breast bone is prominent and your breasts point to the sides, that's the way nature made you and nothing a plastic surgeon can do will fundamentally change that.
Your surgeon may try to move the pockets towards the center, but that sets up three possible problems.
First, if they divide the muscle along the breastbone, then you will see visible implant edges and will have an uncorrectable deformity.
Second, if the implants are moved too far towards the center and the nipples are wide, it will make the nipples even wider and more off-center appearing relative to the breast mound.
Third, there is almost nothing to do to keep the implants towards the center in a chest that has the shape I think you are describing.
Another common surgeon and patient error in logic is selecting a larger implant in the hopes it gives cleavage. It just looks more fake, and the wide distance between the breasts looks all the more odd because of the larger sizes of the breasts, which will progressively fall to the sides.
The implant must be properly placed under the breast; it cannot be located to a particular position on the chest wall. Accept your basic configuration; do not try to shift the implants too far to the center; and select an implant that fits your breasts on objective, quantifiable tissue-based parameters.
I've given you more specific advice than I ordinarily would for a specific situation, but you describe a pattern that I frequently see.