How does using a DC-DC step down converter save power?
Sensors typically do not consume much power. Since the current and thus power is usually drawn from constant current there is no input power savings to step down or LDO regulator to a lower voltage.

Define your design details for a measurable answer.

P.S. Why custom? when there are great 3 terminal IR sensors already that work down to 2.5V like Vishay/Sharp TSMP1138