I think the owner bonus only boosts the effects on the weapon. So like if your weapon has a 10% bonus damage against cavalry, with the owner bonus it will be 15%. I haven't actually used an owner stone yet, so I'm not 100% certain, but I've found multiple copies of the Moon sword, the weapon that 10 Feet of Blue uses. Her Moon Sword has the owner bonus and its base stats are exactly the same as a different Moon Sword without the bonus.

oh bummer

btw, do you know if there's a list of legendary weapon/armour effects somewhere?

I haven't seen any kind of list. It would be useful to have one. There have to be hundreds of different effects. I've already done a long playthrough on the previous version and I'm still seeing new effects that I've never seen before

So I just used an owner stone, and I was wrong about it boosting the weapon's effects. It doesn't touch those at all. What it boosts is the stat bonuses that the weapon gives you when its equipped, which is probably a much better bonus since the effects are kind of useless more times than not.

I used it on the Featherless Arrows. They were originally giving me 2 AGI, 1 Power Throw and 50 Throwing Prof. After using the owner stone, its now 3 AGI, 2 PT and 75 Throwing, so not too shabby after all.