weapon effect is %-based, thus the amount of stat you have will decide how effective it is. the priority works this way, fill your weapon slot first > max level all of it > once maxed search for assault type weapon skill to fill all your slot > start limit breaking > once max limit break > start lvling up the weapon skill (assault first) (this is assuming you have same weapon element with ur KH). if you dont have same element SR weapon, prioritize this... highest rarity > element matching until you get the SR with your choice of element.
R gave 0,5% at lv1
SR gave 3,5% at lv1
SSR gave 6,5% at lv1
all equipped eidolons stats will go to ur character regardless of element.