To let players with high thievery really shine, just make DC+10 = 2 successes.
Reducing time with greater success is a really good idea that fits well with some encounters. It would fit well with skill challenges, but they didn't really integrate the idea of time into them.
One of the big advantages with this is that the only penalty for "unskilled" attempts can be the loss of time. So success can assured, but the time to achieve it in can vary.