Get a gps and recheck your speed. That seems a little hot for a 150. The difference your noticing in speed could be the angle at which water is entering the pickup.
Chine walking is a sign of it getting aired out and on the pad,,,ready to really start running,,,unless your overtrimed. Pardon the comment, but its a mainly driver error.
It will probably take some engine mods to get into the 70's.