Apparently my previous post was too terse, causing some misunderstandings. Let me expand a bit :
Damage caused by a projectile is proportional to its kinetic energy, which is mass times speed squared (m*v^2). Thus a 40/60 reduction in speed corresponds to a 1600/3600=0.44 (44%) kinetic energy at moment of impact compared to the energy of the arrow when it leaves the bow.

But, as Sextus pointed out, we're taking this a bit too far. Back to the subject, it appears that at least some sources believe that firing at different angles was used on the battlefield.

From wikipedia :
"In combat, they would often shoot two arrows, one on a high trajectory and one on a low trajectory. These two arrows would hit the enemy simultaneously from two different angles, making defense difficult."

From ninjacops.com :
"A technique where an archer shoots two arrows at one time would bring down the enemy's defenses. One arrow is shot on a high trajectory and the other at a lower trajectory thus hitting the target on two different angles. "