Very good point, measure everything! One way to start trying a quantitative approach would be to conduct normal interviews with the same candidates and compare

Very good point, measure everything! One way to start trying a quantitative approach would be to conduct normal interviews with the same candidates and compare the results. If you already have a decent interview process there should still be some similar results, indicating you are on to something. Not as good as hiring them all and evaluating performance, but the real world sucks