The -15% damage is additive with Hornet Strike and other mods that increase damage. Combining a max rank Anemic Agility with a max rank Hornet Strike (+220% damage) will lead to a total damage bonus of +205%.
The net damage loss from adding Anemic Agility to a weapon with a max-rank Hornet Strike is only around 4.7%.
Alongside a max-rank Hornet Strike, Anemic Agility represents a 5.3% DPS increase over Gunslinger, although for some weapons the increased ammo consumption and frequency of reloads will offset the modest gain in total DPS.
On release, Anemic Agility was able to drop from GrineerBallistas, which made it the first Corrupted Mod to drop from an enemy unit. This was unintended however, and Anemic Agility was patched out of the Ballista's drop table in Hotfix 15.7.2 (2014-12-19).