For the record, that Archer test shows that stats did not cap at 280, but gave diminishing returns. Furthermore your own tests show increasing Str by +20 gave only an increase of damage by 3 points of damage. You also did not show what exact enemies you used or how many of each. For the most accurate test, you need to use enemies of all the exact same type, of the exact same level, and at least 500+ kills to negate any form of variance.

However, for all those wondering to use, use what gear you feel is best and makes you feel you are doing your best damage. Don't worry too much about increasing your damage to as much of any miniscule amount you can, it will kill you and may make the game not fun haha.

For my own personal gear(I'm still working on, all materia x2/3 melds), I am aiming for gloves with atk+ materia, Felt Trousers with Str/Vit materia, Head piece with Int materia, and Body piece crit+ materia. For feet, well it doesn't matter much. You can either use Element potency+(which itself is debated), or evasion materia. Other slots I'm not too concerned about, probably going to go for some enmity- belt so when Mnk comes I don't get myself killed doing 1,000 Fists/Kick attacks/Element Fists haha.