===================== My 25mm (95mm long) auger bit (Screwfix) projects 2.75" (70mm) from the drill chuck. Cutting a 25mm spade bit to the shortest feasible length would leave a minimum projection of 2.625" (67mm) - a difference of only 1/8" (3mm).
Lengths are proportionately less for smaller sizes of auger / spade bits, so there's not much to be gained by cutting a spade bit.
Cic.