Anyone being able to get the Mythic Wear if put in that very specific context doesn't automatically make it a weaker boost than being Summoned as a Grand.
Amaterasu was compared to the one behind the singularities, not Grand Servant Solomon. She's a threat on the same level as Goetia and his incineration, meaning she's far better than every Grand Servant which appeared until now.
Even then, people like Solomon or Quirinus are special cases that were already absurdly powerful by themselves (Quirinus doesn't even get a boost from being summoned as a Grand, it's just the only way to cointain his whole saint graph). Super Orion would be a better example of the average Grand and how much of a boost it is, but he doesn't really have feats without his special bow made in Atlantis so it's hard to tell.