My son had an 8:30 report last year. We left the hotel about 7:15...there was no backup at Stony. We parked in the waaaaaaaaaay back of the lot by the stadium, as directed. We walked and walked and walked to the front where we waited until all the 7:30 reports left on the bus to Ike. Probably around 8 or 8:15 we boarded. Upon arrival at Ike, we joined a line longer than any Disney ride we had ever waited in. Shortly after 10, my son was ushered in for the briefing and goodbye.
So, given that, you can really arrive whenever you'd like...you're going to be waiting somewhere or everywhere. For us, there was just too much nervous energy in the room to sit around and wait to leave, so we left for WP earlier than necessary. You just never know.
If you are going, there's no need to send your son on the "meet in NY bus." Make it a family time...we are glad we did.