Ok I've got screens of some different +bows.
This one I forgot what it was. It was either +2 or +4, but if you say that +2 has no effects, then it has to be +4. It was definitely not +3 though.
This is +5.
This is +6.
This is +7.
Not sure about this one, it looks like a +7 too but it looks brighter for some reason.. maybe because it's an extra close up.
I don't know what + this is but obviously it's one of the higher ones. So pretty!