The System rods were and are a benchmark in fiberglass fly rods. That doesn't mean they will be a favorite for everyone, for instance, as Magic indicated above., "fast" or what I think of has fast progressive. In the case of my System 5, I very slightly prefer its more supple progenitor, my Fisher 8' for 5. But all the System rods are consistent in type/action. They were perfected enough to be more than likeable for the length/line-weight and techniques of their time; thus, whatever minor dislikes any one user finds (whether in fittings or casting/fishability) tells that angler more exactly what refinements to seek in a slightly different rod, contemporary or "vintage." The previous owner mentioned above, for instance, preferred a different reel seat, whereas I like the one provided exactly as it is. (The owner of the modified rod could easily find and install a standard seat if preferred.) Or I could easily change the System 5 grip to my preferred shape, but the factory version is quite suitable for me, so I haven't bothered.
As for the reel seat, much as I like Rodon hardware with a cork insert, I like the factory seat (a standard component of the time, still easy to find) just as well and have used that model on several other rods, so I wouldn't change a thing. You can easily customize a System rod to taste, but improving it would be difficult.