Spool is like having a locked front one-way. A little less steering initially but gobs of steering throughout the turn.
The carpet we are racing on is med/high traction but it is glued down.. I would not recommend on carpet that is not glued down as it tends to bunch up under heavy braking and turning forces.
spools will greatly improve traction I use it exclusively on asphalt..you get the benefit of a oneway with the braking action of a diff.
With a diff you lose some power and its is slow through transistions. thus faster lap times. than diff