Hornady has published load data for the 500 S&W Magnum and Titegroup covering bullets from 325 gr right on up to 500 gr and pressures ranging from 22.500 psi on right up to 50,700 psi with loads ranging from 11 gr of powder right on up to 20 grains. Sounds safe to me.
OK, look at the Hodgdon data for a 500 gr bullet
13.0 gr =40,200 psi
15.0gr=50,000 psi
Gain in velocity is barely 100 fps
So if the op wants to try and see 15 grs of powder in a 500 SW case, they more power to him.
There is just not much margin for error or wiggle room using a fast powder, 2 more grains ups the pressure by 10,000 psi!!
Max psi is 60,000 psi
Its a 500 magnum not a 38 special!