You would probably see 10K+ with a 16x4. It's not as much load as a 16x6 or 15x8.
I'd bet the guys saying they get 10K+ are using a 15x6 and not a 15x8.

For your 7.7lb plane, I'd certainly try a 16x6 Xoar, JXF or APC sport. Those should get you the best static thrust and reasonable top speed.
They will also slow your down-line speeds. Not all mfg's props are equal.