It's never been stated that MS acts like a 4th tomoe of a Sharingan(i.e. increases your sharingan's abilities). All it does as far as we know, is gives you access to special jutsus. I don't see a problem with Sasuke defeating Itachi with just a regular Sharingan. And even if MS does do what was stated, who said Sasuke's CS form was at a fixed level? It has probably improved by a lot. Or Sasuke has improved so much, that the CS form just makes him that much greater.

Anyways... If I have any problems with this fight, it's the fact that the little brother can beat the big brother. But we don't know that yet.

I just really hate how the older generation doesn't seem to improve much at all.... Considering most of them are still young.