Pretty sure both of these are wrong, at least cmp clearly said that the latter simply isn't true (I have to say, despite my impressions).
My impressions as well. My damage with normal arrows varies widely, from almost no damage to 1 shot kills on certain targets and the difference is always the speed. But certainly what we
do know is that the difference between initial missile speed and relative speed upon impact is what gives you your bonus speed damage. The greater the difference, the greater the damage bonus. That is why slow missiles like throwing spears vs charging horses are so effective, especially against the Courser. That is why you will do more damage against someone who is charging you than from the side or running away. We also know gravity is a factor in the speed bonus, as it has been stated that firing from above to someone down below will give a speed bonus. The rest is physics and what is called into question as to whether the engine models it or not. A heavier arrow propelled at the same speed as a lighter arrow will have more energy behind it and thus do more damage. As I said, cmp would have to confirm whether the greater weight causes a slower launch and thus is factored into the speed bonus, or if the effect is applied after the launch and so is not factored into the calculation, or even whether the speed bonus is based upon initial launch speed or the speed of the missile at the time of impact + the speed of the target. Also, whether the physics engine factors in weight when determining energy imparted or if it is straight numbers off a spreadsheet based upon the initial values in the weapon stats.