Hi Rusty. I also have a 2012 Merc 150 Fourstroke pushing a 2102 Trophy. Had same prop as you (Alum. 17P) which pushed the boat to 39 mph. There was considerable slipping of the prop upon acceleration but it would catch up in a short time after planing. I went to a stainless prop with 15P and eliminated most of my slippage. The fact you state when you back off the throttle and the rpms drop and the boat then catches up would lead me to think you may have an issue with the flo torque prop HUB located inside the prop. saltr.