@kath00 - my ds sent in an arts supplement (music). You are correct. The supplement gets reviewed first which is why it has the earlier deadline. I guess I’m not understanding your question. Sorry 
@bluewater2015 posted the link for the supplement (I see it is now called a portfolio). It appears from that link that a letter from an instructor is now required which was not the case when my ds applied. Pay particular attention to the specific requirements and follow them. It appears from my quick glance that dance is limited to a 7-minute performance. Do NOT exceed that.
Submitting a portfolio is suggested for high level performers, which your dd clearly is. I do not believe that performing artists are “recruited” in the way one typically thinks of that. Her portfolio would be reviewed by arts faculty first and any comments by them would then be sent along to admissions and would become part of her entire application. This is how it was explained to me by a member of the music faculty. In that regard, anyone who submits an arts portfolio goes through that review process first. How much weight those forwarded comments would carry and how influential they are in admissions depends on what is said.
If Stanford is your daughter’s first choice, she should apply REA. Her exceptional dance abilities will certainly be helpful. No one can predict if they will be a tipping point. That said, her ACT is low for Stanford. Can the dance offset that? No idea. Has she taken any Subject Tests yet? Many students who apply have straight A’s, lots of AP exams with 4s and 5s, lots of extracurriculars and tons and tons of something they are passionate about at which they at exceptional. This makes her a typical Stanford applicant. I have no idea about the impact of legacy.
My ds was deferred REA and admitted in the regular cycle.