I don't really get the part where the DC is decided by your own skill. Shouldn't it just be very hard, but if you're very skilled at it, become less dangerous?
I noticed this with regular teleport as well. It was just as easy to teleport with low concentration, as it was with high concentration. That strikes me as odd.
As with the Shadow Walk formula
(Concentration -4) or Search + D20 + Shadow Adept levels VS Search + 2D6 +2d3 +-bonuses/negatives
This would mean that if you leave search at 0, you'll have the most chance of succeeding (as ElderWitch said). Perhaps I'm over-simplifying now, but wouldn't something like this be a lot more logical?
Teleport
DC35 +/- bonuses / negatives VS Concentration + D20 +/- bonuses / negatives
Shadow Walk
DC40 +/- bonuses / negatives VS ( Concentration - 4 OR Search ) + D20 +/- bonuses / negatives
Shadow Walk a higher DC, as it has a higher risk, but also has more ways to higher your roll ( or lower the DC ).
This way teleporting and Shadow Walking becomes something that is only meant for the greater mages, or those with an exceptional concentration.
Just a thought
If this is something you guys see merit in, I'd be willing to think out the actual formula, if you want. I'd just need the piece of code and I could work something out.