Insistent Benefactor trait gives you a +2 bonus on caster level checks for bypassing spell resistance when you cast a "harmless" spell.
From what i can find, a harmless spell is any spell that includes "(harmless)" in its save or spell resistance sections. So that would mean armed bullet is a harmless spell, as detailed by "Saving Throw Will negates (harmless, object); Spell Resistance yes (harmless, object)"... But named bullet also makes you beat the spell resistance of a creature hit by the named bullet later in the spell text.
So do you still get the +2 bonus to beat spell resistance with the spell or not?
I have no plans on taking the trait myself, I'm just curious.
Insistent Benefactor would give you a +2 on your SR check to affect the bullet/ammunition you touch when you cast the Named Bullet spell. You wouldn't receive any bonus from Insistent Benefactor on overcoming the target's SR when you shoot them with the Named Bullet though.
Spell Resistance
Spell resistance is a special defensive ability. If your spell is being resisted by a creature with spell resistance, you must make a caster level check (1d20 + caster level) at least equal to the creature’s spell resistance for the spell to affect that creature. The defender’s spell resistance is like an Armor Class against magical attacks. Include any adjustments to your caster level to this caster level check.
The spell resistance entry and the descriptive text of a spell description tell you whether spell resistance protects creatures from the spell. In many cases, spell resistance applies only when a resistant creature is targeted by the spell, not when a resistant creature encounters a spell that is already in place.
The terms “object” and “harmless” mean the same thing for spell resistance as they do for saving throws. A creature with spell resistance must voluntarily lower the resistance (a standard action) in order to be affected by such spells without forcing the caster to make a caster level check.
The purpose of the Insistent Benefactor Trait is to help you land heals and buffs on your party while in combat at the mid-late game level when SR becomes a thing, because you don't want them to be wasting their Standard Actions to Lower their SR for an entire round.
It's not meant to help you shoot someone with an enchanted bullet :P