If I do:

Function FlyUp()    Ref.PushActorAway(Ref, 0)    Ref.ApplyHavokImpulse(0.0, 0.0, 1.0, 20000)EndFunctionEvent OnEffectStart(Actor Target, Actor Caster)     Target.FlyUp()EndEvent

faster than just writing out it all?

No, it's slower because of the added overhead of a function call.

The language doesn't allow functions to be declared hidden, this prevents the compiler from doing inline optimization's.

If I had something like this:

Event OnEffectStart()    if (Utility.RandomInt(0, 99) > 0)        ;do something    else        ;extremely large amount of code    endif    ;some more code so can't use a Return earlierEndEvent

would having a function make any difference?

Functions won't make anything faster, though that isn't an issue -- they weren't meant to do so.

Instead functions ...

• allow code-reuse -- no buggy copy-&-paste.
  
• are callable from external sources -- scripts can call other scripts (that's how you can access properties).
  
• are named -- improves code readability.

Not much. Functions are best used where you need to use the same bit of code in multiple places, particularly if you need to access the code from outside the script.