Actually it doesn't just boil down to the math becuase there is no way to determine what damage each attack will do or how many hit points a given target will have to figure out how effective those extra 10% of hits might be or might not be.
You say Kivan will do more damage with his extra 10% than Immy will - I say you do not know that becuase you don't know how many hit points his enemy has. If the orc has 10hps and he does 8 with his first hit the second hit can ONLY do 2 points so your "math" goes out the window. Why? Becuase every battle is different every time you play becuase every hit, miss, save, non save, amount of damage taken or given is done by a DICE ROLL. So what was a +8 hit this time is a miss next time or a +2 hit or maybe a critical.
how many of the party is atacking the same creature or how many kill creatures on thier own did that enemy get held or make its save did Kivan get charmed and start shooting at your mage or any number of other variables in ACTUAL GAMEPLAY.
So the FACT is your numbers do not necessarily reflect the FACTS when it comes to REAL gameplay anymore that most of the politicians so called facts and statistics have any relationship to reality. Its just MATH.
A mathmetician once said- figures don't lie but liars figure...
For my money I'll go with my original statement: Why have one superb archer and one mediocre one when you can have two excellent ones. Both are important to the game- both will attack and kill enemies - if Kivan is facing one and Immy is facing one Kivan will kill his whether it takes two shots or three but without the bracers Immy might not kill hers and with them she might. Its not math its just common sense. Unless of course you think Kivan should just do all the killing and Immy sit back and pick her nose. Personally I like to have ALL my characters involved even my mages throw rocks at the enemy when they run out of spells and if you want to find out how bad THEY are with ranged weapons try an all mage game sometime...