Question: what studies are out there regarding substrate heating?

Has anyone ever set up two duplicate tanks with one difference between them:
one tank with, the other tank without cable heaters? How about something
related: 1/2 the tank with a cable heater, the other without? I am wondering
what the real benefit of these rather expensive accessories are.

If there are no such systematic studies out there, anyone have suggestions on
carrying out such a study? What things would you need to be wary of?

Thanks in advance for any information you can provide. -Bruce Geist