The newest prop chart available to the public is for 2010. The 300SLX for that year has 14" X 26" and 15.25 X 26 props which is pretty close to what your boat was delivered with. At WOT does your boat turn up to rated RPM's and not over, or are WOT rpms over or under the rated speed?
At any rate, this is a new boat and it should perform up to Sea Ray's performance specs. I cannot find the speeds for your boat listed on either SROC or searay.com in the brochure, specs or performance pages, but if you are not happy with your performance, this is a new boat covered by full warranties so you need to talk to the service manager at your local dealer and ask his help is figuring out and correcting your performance problem.
Good luck with it and congratulations on a beautiful boat...........
fwebster
