i've noticed fantasmic crowds tend to vary and don't necessarily match the overall crowd level, especially with the new projection show at the chinese theatre starting at the same time. but it isn't a matter of if it'll fill up, it's a matter of how fast it'll fill up--arriving 10 minutes late, you almost definitely won't get seats. and it really wouldn't be a pleasant experience cutting in during the middle of the show, anyway... the star wars fireworks start at 9:30 so you might just want to catch those instead