Reduction in attack speed can be countered by Fury. With a max of each mod, it will result in 100% more damage and 10% faster attack speed. Quickening can also be added to completely negate the attack speed reduction with its 20% attack speed bonus, though with penalties to Channeling Efficiency. Can be further countered with Berserker, but only recommended on melee weapons with a decent (10% or higher) critical chance.
When maxed, spoiled strike gives you +100% regular base dmg and -20% AS. So with this formula : Dmg * (1 + Mod) * AS > Dmg * (2 + Mod) * 0.8 * AS, which is true only when Mod > 3, it isn't worth using it, if you already have +300% base damage mod on your weapon.