Wait, do I get you right ? You want to say that FLB Dark UE Axe (15% + some assault and 15% HP) is not good ? Really ?
According to JP wiki Stingers are rolled separetely, if I calculated it right (which I may not) avarage damage is the same as cumulative though (not including damage caps). So no, relative damage increase is lower the more Stinger you have.
In overall, unless you have some fat strong dual weapons I can't see a reason to pass on FLB UE weapon (aside from not enough materials to FLB/enhance), they are after all at least 15% assault (one percent short of large assault) and something.