Correct me if Im wrong, but dont act2 hell mercs have higher base IAS than their normal counterparts? (or something like that?)Until 1.10, mercs had a level cap when hiring them, being rather low in normal mode. Those from normal mode had (and still have) the best base stats which led to the misconception that it's because of the leveling process. From 1.11 on, the level cap has been lifted.
With other words: No, but it's better to hire them on normal if you have a choice.