I find that things are a lot better than they used to be. If my memory is not entirely faulty, or if I'm not confusing the game with SP:WAW, for a long time infantry would pop off reaction shots to tanks even if they had nothing that could hurt them. This considerably slowed action down to the point that I always hated advancing.
The op-fire filter is not essential against the A.I. but it does make things quite a bit easier. Against a thinking human player you will absolutely want to make use of it to prevent giving away a unit's presence until you are prepared to spring the trap!