Changing it to level+wis wouldn't involve changing very many DCs at ALL, especially if you add in a couple feats that give you a boost to your bending bonus. Or a class feature that gives you a small boost every so often, fills in the dead levels too.
You realize that 2d4 is better damage than 1d8, right? And that the difference in average damage between your max and the original is 3.5, right? Are you honestly going to increase the DC by 25 just for a +3.5 average damage boost? Also, I gave more than just the blast damage suggestion (though I think that one is particularly nice: it gets stronger, it already has a progression we can key it off of, and it doesn't needlessly increase the DC), including one for +1d6/+5 DC. Which gives you +17.5 average damage for +25 DC. Not a meaningless amount. Small, but not meaningless. The other suggestion was the increase in dice size, as Fist of the Forest, Empty Hand Mastery, and Initiate of Draconic Mysteries does for monk unarmed damage. This one is considerably more powerful, as damage dice size increases exponentially. The final suggestion I gave was the dealing blast damage and having the increased DC increase the amount of cold damage it dealt as well.
What is making it do more damage is the better skill you have, thus able to make a better "sword". The dealing blast damage plays off this really nicely. Another thing that makes it deal more damage is it getting colder. Another thing that plays off the blast damage nicely.
EDIT: I'm not calling out the brokenness of competence rings in non-canon, I'm pointing out the various problems with skill based mechanics, most of which apply in canon as well as non-canon, and one of the biggest offenders is only available non-canon. Skill Focus still exists in canon. See my arguments above for details. Also, Truenamers sort of give skill based systems a very bad name.