The deflection bonus is maxed at +5 as the augmented version does not increase this.
Augmented Spells, Shield of Faith says:
Add half your tier to the spell’s deflection bonus
Shield of Faith
...(maximum +5 deflection bonus at 18th level).
For example if you cast an Augmented Shield of Faith as a level 10 Cleric with the max mythic rank of 10, the deflection bonus would be 3 (regular) + 5 (mythic) = still 5 total.
The rule can be found under Effects of Mythic Spells
Unless a mythic spell’s description says it improves, replaces, or upgrades an effect of the non-mythic spell, or says that it creates an effect instead of the non-mythic spell’s effect, it retains all the effects of the non-mythic spell in addition to the effects of the mythic version.
Supporting this is Augmented Anticipate Peril which specifically calls out the increase to the maximum bonus.
Add your tier to the spell’s initiative bonus and the maximum initiative bonus the spell can grant.