IIRC, the reason slaying ammo is so powerful is that critical hits on missile attacks can do 2-6x damage (plus blessing bonus, etc.), whereas melee criticals only do 2x damage. While arrows and quarrels have low base damage (1d6 and 2d6, respectively), you can still get some pretty large bonuses to damage through mastery, Dx, talents, etc. So a slaying arrow could easily have a damage of 1d6+30, which can get up to a 6x multiplier on it.
I don't think it would be a bad idea to submit this as a balance RFE. There's no particular reason why missiles need such a massive critical hit modifier.
Hoping to win with every class, doomed. Archer, Barbarian, Bard, Beastfighter, Druid, Elementalist, Farmer, Fighter, Monk, and ULE Priest down.