not sure how siping would work on your dirt bike but I have seen it come and go on truck tires. One year it is the greatest thing since sliced bread and the next they discount it . They only claims I have heard that it is supposed to do is to prevent wear on the area of a tire that is scrubbing the ground. IE, the outside edges of a steering tire while going around a turn.
It has been my experience that although the edges of a steering tire will last longer from wearing bald the siping has peculiar wear to it. The leading edges of each cut wears down<like the rounding of the leading edge of a knob in your knobby> and can produce a slight vibration not a shake or shimmy just an annoying little buzz that gets transferred up thru the steering wheel and lots of road noise at speed.