Modern bowyers who still offer their bows either way, with or without fast flight, like Schafer, will do some extra work on the limb tips for their fast flight option. I would guess that the fast flight option adds a little weight to the limb tips, which would take away some of the performance if you didn't intend to a use fast flight string. Alternatively, they feel like they need the extra support in the limb tips if a fast flight string is going to be used.
It's often possible to get away with something a product wasn't designed for, but I have to ask myself, "why?" I have 3 Kodiaks from the 50's, 60's, and 70's, and I would never shoot them with anything other than a dacron string. I bought them as representative of that period of time, and I want them to perform as they were designed to perform.