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You might want to study differences in PsyD and PhD clinical psychologists. The thesis per se doesn't seem to be the question but the focus on research. A PsyD dissertation generally does not involve the conducting of an original line of research while the PhD does. I can't think of a better way to equate your groups at the moment.

As a comment I'd like to mention that the benefits of a scientist-practitioner model reach beyond the immediate and are best seen long term, as the practice of psychology evolves. Someone trained only in therapeutic techniques is trained in a stagnant psychology and is ill equipped to understand new research that motivates changes in the field. They may change but the decision to won't be as well grounded in science and will be more based on their clinical judgment.

If you've ever been to a clinical conference where the budding field of clinical neuroscience was endorsed by a speaker through a careful presentation of (astoundingly powerful) research and logical argument you'd see an embarrassing number of practitioners making serious scientific errors dismissing the idea. And it's not just a bias against new but an inability to make sound scientific judgments. The ridiculous case of Autism and vaccines and how many psychologists actually endorsed that research highlights how we need better scientific training. A training that many an experienced scientist can tell you really only comes through practice.

If you come up with a better way to produce a clinician on the day after they graduate then you haven't necessarily come up with a better way to train a psychologist.