"Those 303 million short shares are telling investors that the stock is overvalued, at least in the short run."
Yes, but SIRI has been shorted this way since 2009 and unless the shorts were and are very nimble they have been burned time and time and time again.
Why is it different now? Because the "Great and Powerful Goldman Sachs" downgraded? I guess maybe, but even with a downgrade from the Great and Powerful, I would be very concerned about shorting a company that is profitable, growing, and doing a $2B buy back.