[Autumn]

Spacing has always been very important in any fight vs an Evoker, almost always has it been very important to not simply walk into their effective range or be met with death, that much would apply either way, a 25% damage cut as Chaos brought up doesn't sound nearly as bad as 50%, keep in mind that Throatopener has always been the last ditch effort towards silence.

My statement before was rather exaggerated, but I still think with spacing you can avoid an evoker's onslaught with proper placed silences.

[Raigen.Convict]

After considering the counterarguments carefully a 25% magic damage reduction seems optimal for throatopeners immunity as even from an I.C. standpoint it makes sense drinking a sudden potion to help you focus wouldn't immediately put you at full power.

[Snake]

A -25% damage debuff while the silence immunity is in effect sounds fair enough.