I haven't done stay and scream, but I did do an AP night last year. I do not think stay and scream lets you into any houses as early as the AP nights do. My take from last year was that they kept us in the KZ and opened the two houses there at 5 PM. Last year, we had finished those two houses before the third house opened and we were kept in the KZ until the third house was opening.
What I was trying to say in my earlier post was that it is possible that stay and scream (I assume the group by Finns) might have been able to start around the time they let us out for the third house....when we were let out of the KZ, we just head to the 3rd house that was mentioned for the AP early start, but we did not try to see if would could have headed to the sound stages at that point instead of going to the house by MIB as it was still before 6:30 at this point in the night.
Anyway, from what I experienced last year, the bennifits of the AP event are that:
You can get a friend in early who does not have an AP/day ticket (they still need an HHN ticket and the RSVP)
When the park closes at 5 PM, they let you see the two houses over in the KZ
The third house opens a little later, but still prior to 6:30, so you get to go through three houses that are near by each other prior to the event starting, then I guess the logic is you can see the first showing of Bill & Ted while still in that area of the park.
I feel it gave us a great head start, after the third house, we just continued to the Disaster queue house and then around to the sound stages.
For us, it worked out great as we did go through three houses and although I was not checking the time, I am pretty sure we did those three by 6:30.