Even if you don't upgrade them, 10% chance to do extra attack is still bullshit powerful. If you are lucky, you can get double cast really often and thus significantly increase the dmg you dealt.
However the upgrade is just chance increase to 25%? Wouldn't say it is bad, but for item that expensive, I'd expected something like
10% chance to do extra attack and 5% chance to do another extra attack which can proc from the first extra attack. But eh, as expected from a game which developer cannot handle 3000 top players data and just promptly lose them.