There are many posts like this one which associate First Strike and the Agility Stat and the Initial Counter Value specifically. A max Agility Stat is ubiquitously called out as being inferior to First Strike.
The question however is: "In spite of it's inferiority can a max Agility Stat provide a sufficient Initial Counter Value to overcome an ambush?" I can empirically say, yes to this, as I have been ambushed by monsters with a low Agility Stat and had my characters, which didn't have First Strike or Initiative weapons, get a turn before the ambusher. However, since we are working under the premise that First Strike is superior to a max Agility Stat, it must be noted that some monsters have the First Strike ability:
- Varuna will always open the battle with Haste
- Kottos will always open the battle by switching into its Counter status, signified by the raising of its fists
- Great Malboro enemies fought in the Omega Ruins will always ambush the player using Bad Breath on their first turn, however, a player's First Strike ability precedes this
So even a max Agility Stat, providing the best possible Initial Counter Value, will still be inferior to the Great Malboro's First Strike. I believe you see that called out in posts like this one, which correctly surmise an answer to this question:
Agility in excess of 170 only effects the Initial Counter Value, which is pointless against an enemy with First Strike. Only your own First Strike weapon will allow you to take a turn before said enemies.