We had a 5:45 PS last year and got out in time to spend a few minutes in Tomorrowland before the party officially started. We actually got seated closer to 5:30, though. But LTT didn't seem to get busy until well after 6pm. Maybe the restaurant is just laid out in a way that disguises the crowd.
The PS's seemed to get snapped up quickly, so plan on calling ASAP.
And have fun at the party! We've been the last 2 yrs but can't make it this year.