I traveled to the US and back multiple times for work while I was waiting for my PGWP; never had a problem at the border (and unlike you, I didn't even have a valid study permit any more).I applied for my pgwp on 11 October, seriously wondering if to leave the country for a vacation while I wait, still have a valid study permit...thoughts?
CIC's guidance on this is pretty clear. Just make sure that you have a valid eTA/TRV (as applicable) when you return, and proof that you have applied for the PGWP.