You shouldn't use SOG or GPS for speed. The best indicator of how fast your lures are moving through the water is downrigger wire angle.
Using SOG/GPS speed only works in still water. If there is any current, you will be whipping your gear one direction, and hanging limp the other direction. The blowback angle of your wire is the best indicator of relative water speed over your lure, and adjusting speed to keep that angle constant will ensure the lure action is constant.