My own experience of FX1 with the Iodide solution was that with FP4+ and HP5+ it seemed to do.... nothing.
Nothing bad happened, but the developer didn't seem to behave any different to a similar metol based developer without the Iodide.
Ditto with FX2
I never tried them with Pan F, maybe that was the one I should've tested?

So, my humble opinion is that FP4+ and HP5+ do not respond like 'old emulsions' to these old acutance developers.

The only developer that I found that seemed to increase the acutance of FP4+ to any noticeable extent and give some prominent edge effects, much more so than dilute Rodinal, was Paterson Acutol. I'd love to know the formula for that, especially now it is no more...

One day I am going to give FX1 one last go and dunk some ADOX 25 in it.