Indeed, this is how they work. The long irons launch higher than the PX standard and the short irons perform very much like the PX. I also have PX (standard) wedge shafts to match the PX flighted. The short irons (PXF) and wedges (PXS) perform similarly – mid launch with a flat arc on top.
That is except for feel. The standard PX will probably not only PLAY stiffer and FEEL stiffer but will actually BE stiffer. The 1st two have to do with the characteristics of the PXF shaft, the latter with the fact that Royal never really made shafts like any other company. In True Temper shafts, a Stiff was a Stiff. That's really not how it worked with Royal Precision. You need to build them to a desired stiffness if you know what I mean.
Long-winded way of saying, yes, the PXF 6.0 is softer - both because of it's design and the fact that it probably actually is playing at 6.0 installed (something you couldn't always say about PXS).