We had an 8am on Crystal Palace and my recollection was that at about 7:55, they brought in EVERYONE who was already waiting at the turnstiles, regardless of actually reservation time (as long as it was before park opening, I guess?). Then we checked in at a podium and we were seated based on the order we were checked in there, NOT our actual ADR time. This was a year and a half ago and this was at least how it SEEMED to me so I would say if you are at the turnstiles by 7:45 you are probably going to have no problem getting in.
And if not, they seemed to take people in big groups. If they make you wait until 8:15 or so, the 8:00 crowd will all be seated already and Main Street will once again be desserted, so either way I think you will get what you are after.