You need to first make sure the motor is mounted at the right height. The flat plate above the prop, called the cavitation plate, should be even to 1/2″ above the bottom of the hull. This could also be a trim issue. The question is do you have a manual lift or do you have power tilt/trim? If you have a manual tilt/trim motor, my guess would be that you have the trim stop, the rod that crosses the motor mount to stop the motor from getting closer to the hull, is in the wrong position. If the motor is not positioned correctly in height/angle the motor will not perform correctly. The bigfoot does have a different gear ratio and it also has a beefed up lower unit. They were designed for pontoons, but they can also be used on boats. My guess is that you also have a tiller boat? A 25 hp on a 16 Laker is a bit underpowered in my opinion, but it should still plain out. I would also throw a tach on the motor and check to see what the rpms are running at wide open throttle. If it is not running at the upper end of the operating range the prop must be changed out with one that will get those rpms. The right prop will also make a difference on how the boat performs. Good luck! 
