The thing is shell speed effects ROF (as does reload times), plus if shell speed was reduced it would be slower from muzzel to target, shell speed drop off is not modeled in game. ie: Gun fires at 700mps, but the shell would travel at say 678mps from muzzel to target. ROF now waits till each round reach's travel end at 678mps rather than at 700mps. Also gravity is modeled in game, you reduce shell speed and it will have a more pronounced arch in its trajectory. Which is rather odd when you think about it, to have gravity modeled on shell flight trajectory but not have any impact on shell speed!
Also since wind is not really modeled in game either you can see why there is no reduction or drift on rounds. The other thing that is modeled correctly is lead. Lead is governed by direction of weapon mount's direction of travel and speed as well as direction and speed of target, as it should be (this looks like the shell has arched to the left or right when fired at a target in flight).