Willamette Valley native plants are accustomed to summer drought, so they will continue to fare well even if the summers become somewhat longer and drier (as they seem to have become since 2015). Native plants from the Umpqua or Rogue areas of southern Oregon are likely to be somewhat more drought tolerant than Willamette Valley native plants just because that area of Oregon tends to be warmer and drier. Plants from central or eastern Oregon originate in a continental climate, so different than western Oregon and so in some cases might struggle with the soils and spring rain we receive over here. 

As far as evaluating plants for drought tolerance, we have done trials of many plants for the Willamette Valley for un-irrigated landscapes over almost 20 years under the name "Northwest Plant Evaluations". You can find more information in the link below. Feel free to write with questions.

https://horticulture.oregonstate.edu/nursery-greenhouse-and-christmas-trees/production/northwest-plant-evaluations