the backerboard isn't about levelling. you'd still need to self level on top of it in this scenario i suppose. the backerboard is just a superior substrate to tile on. especially for floors where the weight of people walking flexes the subfloor. backerboard fights the flex. gives you a more solid floor... fights cracking in the grout, etc.

fair enough... then i'd self level. i think i'd get a solid backerboard base down first.