Sickness holds him back. However, Itachi does have a cool head like you pointed out. I see your argument, but I think Madara has higher chakra reserves than Itachi. The fact Itachi was so cautious, not wasting his movements, means he likely didn't have huge chakra levels like Madara.
It's something I've been noticing throughout the series. The more intelligent characters tend to have handicaps. Their intelligence and high observation is balanced out by lacking something, otherwise they'd be god-tier.