IRM Vs. IAU?

I was just considering what Equip to put on Skorp G, I think rammer and binos are a must, but what for the 3rd spot?

At first I thought IAU as Skorp usually needs fully aimed shots, but then I got to thinking, 5% when accuracy is already 0.29 is really not that much. Probably reducing after firing/movement dispersion value is more more important, right?

Then I thought when is IAU actually better than IRM? Meant to be for redline snipers right? But redline snipers have super high accuracy by definition, so is a flat 5% bonus that useful? Would you put it in firepower slot over rammer to get the 7%? Maybe on lower tiers like hellcat? Or maybe you want change the role of the tank, like sniping heavy that has worse initial accuracy (maybe tiger)? Or maybe in clan wars, with something like strv, when you know your specific role?

Help a noob out…;)