The earlier the time, the fewer guests will make it.
However, since there will be a much larger OVERALL guest population during the dates when the park has the extra-early EMH, expect there to be lots of guests awaiting the opening.
Just know that first hour will be see the lowest number of guests compared to the rest of the day, regardless.
As to when to arrive...
Since it is nearly impossible for guests to accurately judge exactly how long it will take to actually get to the MK gates (variables of traffic and other timing issues... especially if you are driving,) it makes sense to at least PLAN on getting to the MK gates 30-45 minutes before the posted opening time...
and at/near Easter, the MK parking lot can actually FILL UP by early in the day.
If you really want to be SURE of arriving on time for gate opening. Go EARLY, early.
At MK, they will usually start the MK Welcome (Rope Drop show) at 15 minutes before the posted opening time.