Hi, first of all sorry for my bad english.
In pure math terms the SB doesn't change the DOF but that's not important. The DOF depends on the distance from the subjet too so because the speedbooster changes the field of view , you must approach from the subject to have the same shot you would have without the speedbooster. It means that, actually, you have a shallower DOF. (this questions has been answered many times on BMC and on EOShd i think)