I'm sorry but Stanton hasn't been a reliable lefty since before he went to the Mets. Quantrill is just a matter of time before he goes back to stinking the place up. He has looked good for the time being, maybe he just needed a change of scenery and a chance to refocus himself.
Anything's possible, but Stanton's situation isn't as favorable as Quantrill's. Stanton's probably going to be used in less favorable situations (coming in to face a left-hander with runners on) than Quantrill (mop-up or late-inning eater). Also, the Nationals don't have a track history, like the Padres, of turning once-good relievers back into their good-pitching self.