Sanji in his base form, proven can go toe to toe against RS Judge. In fact, Sanji easily break Yonji body which didn't don his RS in their early skirmish. Base Sanji also capable breaking Perospero Candy Wall, which his siblings need RS to done the same. Its not to mention that Sanji siblings is weak, but just to prove that base Sanji prowess is on par as his family which donning RS.
So, by donning RS, Sanji acquired next level power up over his current limit which suits to his character development and give more probability for him to taking on any YC level.