300 killed people is far greater loss than 50 fighters.If 50 of them had MS, then 50 people would already be dead. That isn't a small loss.
I don't think Itachi is stupid. I think it is more like him to try an indirect method of bringing Madara down. By forcing sasuke to become stronger so maybe eventually, he could kill madara for him. he doesn't seem the type of person to go head long into a battle he knows he can't win.
And Itachi is not stupid indeed but killing his clansmen was not going to help. Which means that Madara is lying or that Itachi was idiot. I think i ll choose the first one.