I parsed 151 fights from level 19-25 and have confirmed the +10 accuracy.
Melee accuracy raised 10% while I had this food on over meat mithkabobs. I say +10 acc, because I also had +5 accuracy over the sam and nin in my party and my mithkabob accuracy was also 5% higher than there's. I think that's pretty strong evidence. +5% acc given +5 acc gear over 'normal' characters. And then I use the lantern and go up +10% over what i was with mithkabaobs. I think in this level range that +1 acc = +1% accuracy.