With +18 Fire Potency (+9 from Electrum Circlet (Rubellite) and +9 from a Fire Materia) there's absolutely no damage increase... even a slight one to Fists of Fire. I realize 18 is a fairly low number but the elemental affinity materia have been fairly difficult to come across for me =/

Not to mention it would really suck to double (or higher) meld on your feet just to find out that it's doing absolutely nothing and could've added something else. Maybe once someone does manage to get a higher amount it can be extensively tested. At the moment it seems like the easiest amounts to come across aren't doing anything significant.