You should add to your suggestion that there should also be an increased general rate of fire as well.
The purists in me likes the 4x arrows that Turbine has which corresponds to 3.5, but the problem is Turbine did not implement this correctly. The way manyshot works in 3.5 is you fire 4x arrows in a standard action and then you can use a move action in a round. The move action can not be used to shoot any more arrows. The other option in 3.5 is to shoot the equivalent amount of arrows basically in a Full round action which also uses a round. For all practical purposes manyshot has the same rate of fire in 3.5 as does regular shooting it just gives a character the additional option of executing a move action.
The general rate of fire should be the same for a moving manyshotting character in DDO as a standing non moving archer. I would guess that this is very difficult for Turbine to code though; hence, we do not have it in DDO. I do not know what would be the best way to solve this problem because the answer really resides in the code iteslf.