The 8x4 prop is just too big for that engine. Overloading it isn't helping break the engine in, and may reduce the power you eventually get out of it. 14.5k on the overly large prop and 15% fuel is fine though.

The Magnums like a smaller prop. Break in with a 7x4 MAS, and fly with the 8x3. Throw that 8x4 away. And take a headgasket out if you haven't already, the Mags come with 2 installed. With the 8x3 and 15% fuel, depending on the weather, and after the engine breaks in more (they take a bit of time to reach full power), you should see around 17k, sometimes 17.5k or more, sometimes in the high 16s. Don't sweat it, if you are over 16.5, you'll be fine, the Mag XL unloads quite a bit in the air (well, if the airframe is clean enough anyway). The OS LA turns more RPM on the ground, but doesn't unload nearly as much in the air.