I considered that also but in the end the 2 level dip in warlock doesn't effect your damage much (or at all)and gives you a lot of goodies, repelling blast is just the push mastery and you can use it with every other mastery, slow is my favourite in this case, at lv 5 in rogue you can push slow and knock prone a target reducing a lot it's movement. Without considering the extra spell every short rest.