Siping compromises the surface tread support. In other words, it will make the tire tread squirm on contact with the pavement. You will feel this in the car and it could be very unsettling. If the car is driven long distances on the highway, the heat built up in the tread surface will be greater than non-siped tires and could cause the tread to break off little pieces. The opposite of this is tire shaving; cutting away half of the tread depth to reduce tire squirm and heat buildup for street tires used for racing.
Siping, from a marketing viewpoint, would allow the use of tires not appropriate to drive in the ice and snow to be used instead of swapping to the correct winter tires. The engineer’s view is that it screws up his tire design. The cuts (or sipes) are theoretically small enough (IMHO) to wear away over a winter season. When spring comes, the tire acts as it did before the sipes were added.
As for a fuel mileage increase; no way. It would either have no effect or degrade the mileage.