Regarding the Tomb of Horrors, I think that if the taking 20 is creating a problem, you should develop and examine the search DCs of each trap, and from there customize the DCs a bit. A trap that should be impossible to find can have a DC high enough that it won't be found without using extreme measures. Other traps that "should be able to be found with high skill" should have DCs that are in range for take 20. Traps that have time constraints or other pressure should probably remain within a typical range for your party (requiring some moderate DC).