Obewan,
I think there are far more important critieria for deciding amongst SS alloys than ferrite number. Ferrite is really only an issue in that too low increases risk of hot cracking, too high increases risk in intermetallic precipitation. Within the acceptable range its almost a who cares. If you look at the diagram marswl provided 316, 347, and 308 are very close. Any greater variance is getting into alloys with entirely different applications and properties. For example 2209.
Add to this the fact that FN will be influenced by welding parameters (especially anything that involves varying cooling rate)so the numbers published are only ranges in themselves.
Also, the great majority of SS base metals are fully austenitic at room temp(obviously duplexes, FSS, and MSS and most PH's would be exceptions). They don't have to worry about hot cracking. They worry about hot forming, cold forming etc. An entirely different set of concerns. This besides the fact that their chemistry is purposely designed to ignore ferrite content(obvioulsy except for FSS, etc.), and that the cooling rates after solution annealing is not fast enough to lock in delta ferrite through the austenitic range anyhow. In order to lock in delta ferrite at those slow cooling rates the CrNi ratio would have to be quite high. Besides, what do the base metal guys even care about delta ferrite?
One more thing, take the 347/321 comparison for example. I'd be more concerned about 321 losing Ti across the arc if stabilization is the concern, that is if you can even find 321 on a regular basis, or at all(except maybe for GTAW). Or 347's penchant for hot cracking or knifeline attack, regardless of its ferrite.