It will depend on the park and how close different headliners are to each other.
For example, at Epcot -- the 30 minutes will likely just give you 1 headliner, but also put you in great position for a second.
So if it's a 9:00 am official open at Epcot -- Early Entry starts to let people in shortly before 8:30. By 8:40, you're arriving at Test Track. You're off Test Track by 8:55... at 9:00 am, just as regular guests walk in, you're ahead of them, getting on line at Frozen. You're on a 15 minute wait for Frozen, while a 45 minute line forms behind you.
At MK, with short rides closer together, you can more easily get a 3 ride advantage. By 8:35, you're arriving at Mine Train.. you're off it by 8:47, you go straight to Peter Pan, and you're off Peter Pan by 9:00 a.m. ... and just as other guests are coming in to the park, you're already on your way to a 3rd headliner.
Same as DHS -- You can probably knock out all 3 Toy Story land rides in the first 45 minutes, with Early Entry.