I quite like this concept, just as a small change of direction for the spell.
Moreover, it fits the Eagles as an offensive tactic. By disturbing the enemy and allowing your own army to catch up, they really provide support from above similarly to how modern fighter planes clear up the field.
Besides, reforming the Eagles spell allows for a further connection to the iconic moment of their arrival, in front of the Black Gate at the final hour, as the Messengers of Manwë. I believe we can justify some creative liberty by incorporating Aragorn's charge for Frodo into this spell

As such the spell reconnects much better with this moment that determines the end for one of two ideologies.
I do think the ability needs a bit of refining. Gwaihir attacks more than twice, maybe thrice within 10 seconds, making the active ability more passive in function. Perhaps if the bonus is limited to 2-5 seconds in a small radius underneath him, it encourages players to move him accros the front line (similar to the new King Dáin on his boar).
I would like to see this develop further, and make the Eagles great again!
