Throwing nails at someone is no different than any other Evocation in terms of damage, them being iron means they'll ignore it's toughness abilities, but doesn't grant any additional damage per se. Now, if she spends an action on a Declaration or assessment regarding their iron as an anti-Faerie device, that'd add, but barring that you're completely correct, the Sidhe takes a 'mere' 8 stress hit that ignores it's standard Toughness abilities.