Of course not. Johnny's productivity is going to be severely affected by a few months of 80 hrs/wk. If you don't have enough staff to do what you want in however long, you have to take longer, or hire more people.
But I wasn't referring to that situation. For a given project, there's an optimal team size. Adding people to that team can often reduce the productivity of the team as a whole. There's a limit at which you just can't get the project done any sooner. In that case you need to set a realistic schedule given these limitations.