DH has as much impact on Healer DPS as racial stats. That's not to say it shouldn't be melded, but it relies on the premise that you're able to fully maximise the DPS potential of SCH (or WHM and AST, respectively) to begin with, in addition to coordinating well with your co-healer. If you can't do this, then going Det is most likely going to be the better option.

This Reddit post covers the subject in more depth: https://redd.it/7l22nr