I asked my fitter the same thing a few month ago and he told me that in his experience, he never saw huge improvement by going with a hybrid shaft versus a steel one.
In the purpose of building a driving iron of course. Like, i have the 3 iron. But the distance was very close to the 4 iron and wanted to add distance to the 3 iron and asked about graphite. We tried, and i didnt saw huge improvement with graphite.