Will there be a noticable difference between 250cc/275cc moderate plus implants or if I choose 270cc/295cc moderate plus xtra?

Between these two profiles, the Moderate Plus Xtra in those volumes adds just a little more upper pole fullness long term vs the regular Moderate Plus.  The Xtra help maintain a fuller sloped appearance in the upper pole over time whereas the Moderate Plus can settle slightly more and lose a little of the desired look.  I generally favor the Moderate Plus Xtra these days since they feel equally as soft but still maintain better shape.  I hope this helps.

The difference between higher and lower profiles for a given volume is projection and for a given width is volume (with the high profile implant being greater in both scenarios). The moderate plus is the most commonly used profile in my practice. It allows for the most reasonable volume in a given BWD. In my practice, low profile implants are the least commonly used (widest and flattest). I will typically reserve high profile devices for smaller framed individuals or those seeking a less subtle more dramatic change (the opposite of natural). 
In general, the difference between a high and Mod (+), in reality, is very small (on the order of millimeters).  In my practice, I generally classify profile based upon patient preference. A higher profile provides a more dramatic look (size for size) vs. a moderate (+) profile which is relatively more subtle and understated. That being said the vast majority of patients will select a moderate + profile device (especially those seeking a more natural and subtle result).
I often find it is easier to decide on a profile (rather than a volume), this combined with BWD will determine volume. Trying to match a particular volume to a profile may lead a BWD mismatch. In my practice, smaller framed individuals do prefer higher profile devices. Ultimately, profile choice is a choice between more or less subtlety.
I would recommend more communication with your surgeon. It is his/her job to guide you through this process. Their experience is a resource...use the resource.
As always, discuss your concerns with a board certified plastic surgeon (ABPS).
Donovan Rosas MD
Board Certified by the American Board of Plastic Surgery
Member: American Society for Aesthetic Plastic Surgery, American Society of Plastic Surgeons
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You won't notice much with the additional 20 cc's, but the higher profile will give a little more forward projection. If you are uncertain, go with the larger size, or go back and talk to your surgeon. Your surgeon should be able to advise you.