I hope you don't assume high agility means high APs means more bullets. Agility has nothing to do with shooting unless you need to move into position before firing. Having a 0 AGI merc or a 100 AGI merc in position shooting at targets makes no difference whatsoever. That's because firing costs are always relative. Having half the APs due to low agility means spending half the APs for shooting in comparison. Movement however always costs the same, which means having more APs only helps you move further.